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Neodymium (Nd) isotopes are considered a valuable tracer of modern and past ocean circulation. However, the promise of Nd isotope as a water mass tracer is hindered because there is not an entirely self-consistent model of the marine geochemical cycle of rare earth elements (REEs, of which Nd is one). That is, the prevailing mechanisms to describe the distributions of elemental and isotopic Nd are not completely reconciled. Here, we use published [Nd] and Nd isotope data to examine the prevailing model assumptions, and further compare these data to emergent alternative models that emphasize benthic processes in controlling the cycle of marine REEs and Nd isotopes. Our conclusion is that changing from a "top-down" driven model for REE cycling to one of a "bottom-up" benthic source model can provide consistent interpretations of these data for both elemental and isotopic Nd distributions. We discuss the implications such a benthic flux model carries for interpretation of Nd isotope data as a tracer for understanding modern and past changes in ocean circulation.
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published: 21 December 2017
doi: 10.3389/fmars.2017.00426
Frontiers in Marine Science | 1December 2017 | Volume 4 | Article 426
Edited by:
Catherine Jeandel,
UMR5566 Laboratoire d’études en
Géophysique et Océanographie
Spatiales (LEGOS), France
Reviewed by:
Tina Van De Flierdt,
Imperial College London,
United Kingdom
Vineet Goswami,
Colorado State University,
United States
Brian A. Haley
Specialty section:
This article was submitted to
Marine Biogeochemistry,
a section of the journal
Frontiers in Marine Science
Received: 02 October 2017
Accepted: 12 December 2017
Published: 21 December 2017
Haley BA, Du J, Abbott AN and
McManus J (2017) The Impact of
Benthic Processes on Rare Earth
Element and Neodymium Isotope
Distributions in the Oceans.
Front. Mar. Sci. 4:426.
doi: 10.3389/fmars.2017.00426
The Impact of Benthic Processes on
Rare Earth Element and Neodymium
Isotope Distributions in the Oceans
Brian A. Haley 1
*, Jianghui Du 1, April N. Abbott 2and James McManus 3
1College of Earth, Ocean, and Atmospheric Sciences (CEOAS), Oregon State University, Corvallis, OR, United States,
2Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, Australia, 3Bigelow Laboratory for Ocean
Sciences, East Boothbay, ME, United States
Neodymium (Nd) isotopes are considered a valuable tracer of modern and past ocean
circulation. However, the promise of Nd isotope as a water mass tracer is hindered
because there is not an entirely self-consistent model of the marine geochemical cycle
of rare earth elements (REEs, of which Nd is one). That is, the prevailing mechanisms to
describe the distributions of elemental and isotopic Nd are not completely reconciled.
Here, we use published [Nd] and Nd isotope data to examine the prevailing model
assumptions, and further compare these data to emergent alternative models that
emphasize benthic processes in controlling the cycle of marine REEs and Nd isotopes.
Our conclusion is that changing from a “top-down” driven model for REE cycling to
one of a “bottom-up” benthic source model can provide consistent interpretations of
these data for both elemental and isotopic Nd distributions. We discuss the implications
such a benthic flux model carries for interpretation of Nd isotope data as a tracer for
understanding modern and past changes in ocean circulation.
Keywords: rare earth elements, neodymium isotopes, marine geochemistry, benthic flux
Neodymium (Nd) isotopes (denoted εNd, which reflects the 143 Nd/144Nd ratio normalized to
a Chondritic Uniform Reservoir; Jacobsen and Wasserburg, 1980) are widely recognized as
a valuable tracer for ocean circulation (e.g., Piepgras and Wasserburg, 1980, 1987; Elderfield,
1988; von Blanckenburg, 1999; Frank, 2002; Goldstein and Hemming, 2003; Piotrowski et al.,
2004; van de Flierdt and Frank, 2010). The potential for Nd isotopes to serve as a circulation
tracer lies with the observation that εNd distributions appear to mirror the pattern of global
deep-water thermohaline circulation. These observations support the assumption that εNd is
conservative or “quasi-conservative” in the oceans and will trace water masses in a manner
similar to temperature and salinity (Frank, 2002; Goldstein and Hemming, 2003). Furthermore,
elemental Nd is assumed to have negligible bioactivity, although recent work suggests that
there are important caveats to this assumption (Shiller et al., 2017). However, because of
the concerted efforts invested into measuring neodymium in the oceans as well as within
sedimentary archives, inconsistencies between the observed distributions and the assumptions
inherent in the use of the Nd as a tracer are becoming increasingly apparent. The best known
of these inconsistencies is the “Nd paradox” (Jeandel et al., 1995, 1998; Tachikawa et al., 1999a;
Lacan and Jeandel, 2001). Described in many ways, this paradox fundamentally describes the
contradictory observations that while εNd appears to behave conservatively, the water column
profile of dissolved elemental Nd ([Nd]) appears to reflect the behavior of a reactive element,
showing a nutrient-like distribution (see discussion in Goldstein and Hemming, 2003). In short,
Haley et al. Benthic Control Over Marine REEs
εNd distributions imply a residence time of 103years, while
[Nd] distributions imply a residence time of 104years (Bertram
and Elderfield, 1993; Jones et al., 1994). This discrepancy
appears in paired geochemical mixing models (Goldstein and
Hemming, 2003) and models that attempt to reproduce the global
distribution of εNd (Tachikawa et al., 2003; Jones et al., 2008;
Arsouze et al., 2009; Rempfer et al., 2011). In addition to the
“Nd paradox,” inconsistencies have arisen regarding past records
of authigenic εNd, which sometimes prove to be difficult to
explain when assuming water mass mixing of fixed end-members
(Osborne et al., 2014; Stewart et al., 2016), or show exceptional
disparities between adjacent sites (Stumpf et al., 2010; Roberts
and Piotrowski, 2015; Howe et al., 2016; Hu et al., 2016).
The problem with εNd as a circulation proxy is fundamentally
that while εNd appears to trace water masses effectively, it is
not particularly potent at discerning geochemical processes that
might impact its distribution. For example, it is difficult to
use εNd to determine how a water mass acquires its isotope
signature. Although the pattern of global εNd mirrors that of
thermohaline circulation the cause of that apparent coincidence
is not straightforward. For example, that the deep North Pacific
is a Nd isotope end-member for mixing is difficult to justify
given that no deep water forms within this basin. If isotopic end
members can develop unassociated with preformed properties
of the water mass, then there is clearly a need to explain and
constrain the effects of these processes on εNd, especially for
interpretation of εNd as ancient records of circulation.
Relatively recently, several possible mechanisms for non-
conservative behavior of εNd have been forwarded: boundary
exchange, Submarine Groundwater Discharge (SGD) and a
benthic Nd flux (Lacan and Jeandel, 2005b; Johannesson and
Burdige, 2007; Abbott et al., 2015a). These three hypotheses are
all similar in that bottom water can be altered along the sediment-
water boundary. Although these hypotheses may have been
articulated as distinctly different, the differences among these
hypotheses may be simply semantic or reflect our incomplete
understanding of the processes involved. Boundary exchange, as
described by Lacan and Jeandel (2005b) is typically thought to
reflect an isotope exchange of Nd with net zero concentration
change that occurs between the sediments and bottom water
predominantly at ocean margins (Jeandel, 2016). While this
model is highly cited, the specific mechanism or processes are not
well defined. In contrast, both the SGD and benthic flux models
propose more distinct mechanisms but lack the robust global
data sets needed for verification. All three of these hypotheses
invoke a potential source of dissolved [Nd] to the oceans
from sedimentary fluids; SGD waters are considered here to be
originally fresh waters, while the benthic flux model considers
seawater as the source fluid origin. These latter hypotheses will
differ in the quantity and location of the Nd fluxes they predict
and the εNd that they carry. Clearly, the distinctions among all
three models is nuanced, and again, probably simply reflects the
need for further investigation or even simple definition. However,
all three models challenge the underlying assumptions around
the conservative nature of εNd in the water column.
A more comprehensive understanding of the marine
geochemical cycle of REEs in conjunction with εNd is needed to
build more accurate geochemical models and interpret down
core records with respect to ocean circulation. The problems
arising from the notion of εNd as a paleoproxy perhaps reflect
the superficial, and perhaps data limited synthesis between
analyses of εNd and the rare earth elements (REEs) throughout
the ocean basins. Fortunately, the comprehensive nature of the
GEOTRACES water column sampling will go far to improve our
understanding of water column processes that affect the REEs
and εNd. Here, we discuss published εNd and Nd data from the
Atlantic Ocean, with the intent to clarify our understanding of
these tracers where they appear to be most robust. While we do
not present any new data, we attempt to illustrate that a holistic
view of published REE and εNd distributions in the oceans is
consistent with predominant control from a benthic sedimentary
source. Our conclusion is that processes occurring within the
water column (e.g., reversible scavenging) are likely a secondary
control on εNd or [REE] in the oceans, save where particle
fluxes are exceptionally high, and that benthic fluxes exert
primary control over the distribution of these elements and their
It was originally held that the REEs are input to the oceans
predominantly via rivers, with potential additions via dust where
such inputs are relatively high (Byrne and Sholkovitz, 1996;
Greaves et al., 1999; Tachikawa et al., 1999b; Goswami et al., 2014;
Dunlea et al., 2015; Stichel et al., 2015; also see Bayon et al., 2004
for the alternative view that dust is a sink term). More recently,
the emphasis of REE input to the oceans has shifted to greater
consideration of a benthic continental margin source for REEs,
as we will discuss further later (Spivack and Wasserburg, 1988;
Jeandel et al., 1998; Goldstein and Hemming, 2003; Tachikawa
et al., 2003; Arsouze et al., 2009; Rickli et al., 2009, 2010, 2014;
Carter et al., 2012; Grasse et al., 2012; Singh et al., 2012; Grenier
et al., 2013; Wilson et al., 2013; Garcia-Solsona et al., 2014).
In the open ocean, REEs are thought to be transferred from
shallow to deep ocean via reversible scavenging on particles
sinking through the water column (Elderfield, 1988; Byrne and
Kim, 1990; Sholkovitz et al., 1994; Byrne and Sholkovitz, 1996).
However, the heavier atomic mass REEs (HREEs, such as Yb,
Lu) tend to have vertical profiles that are more “Si-like,” which
is thought to reflect the tendency for greater complexation
across the REE series (Cantrell and Byrne, 1987; Elderfield, 1988;
Byrne and Sholkovitz, 1996). Finally, removal of REEs from the
ocean is thought to be predominantly via metal-oxides, although
the role of organic matter, phosphates, or even carbonates,
is likely to be important (Byrne and Kim, 1990,?; Byrne and
Sholkovitz, 1996; Schijf et al., 2015). This model is considered the
standard of marine REEs. Nd isotopes should also behave in a
manner that is consistent with this model, although as pointed
out previously this does not appear to be the case (Goldstein
and Hemming, 2003; Jones et al., 2008; Arsouze et al., 2009).
Successful efforts to reconcile the inconsistent behavior between
elemental Nd and its isotopes, i.e., the Nd paradox, will likely
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Haley et al. Benthic Control Over Marine REEs
begin with an examination of these first principles of marine REE
The shift from considering riverine point sources to broad
marginal inputs improves model results (Tachikawa et al., 2003;
Arsouze et al., 2009) and offers an explanation for some of the
irregularities observed when comparing Nd and its isotopes such
as the constancy of surface ocean water εNd (7.8 upstream
vs. 7.9/8.0 downstream; Osborne et al., 2014) around the
mouth of the Mississippi river that carries a far less radiogenic
signal (at εNd <11; Goldstein and Jacobsen, 1987, 1988;
Bayon et al., 2015. Budgetary estimates from diffuse benthic REE
fluxes, such as SGD or from sedimentary pore water (Sholkovitz
et al., 1989; Greaves et al., 1999; Haley and Klinkhammer,
2003; Tachikawa et al., 2003; Johannesson and Burdige, 2007;
Schacht et al., 2010; Johannesson et al., 2011, 2017; Abbott et al.,
2015a; Fröllje et al., 2016), suggest that they may dominate the
flux of REEs to the oceans. Importantly, such diffuse, or at
least hard to quantify, sources are consistent with deep-water
isopycnal mixing of εNd signals off margins that cannot be
explained through surface water sources (Grasse et al., 2012;
Grenier et al., 2013). However, while a diffuse benthic source
of Nd appears to better describe the dominant flux of REEs
to the ocean, outstanding questions remain. Arguably the most
important of these is: what drives this flux? Any mechanism
must be reconciled with the remarkably consistent REE pattern
of seawater in light of diverse sediment compositions and
sedimentary/diagenetic environments. As such, it seems unlikely
that simple, unidirectional particulate dissolution can be the
mechanism, because the dissolved REE pattern and εNd would
to some extent reflect such an input, following arguments posed
by Sholkovitz (1993) and reiterated by Bau et al. (2013).
Another supposition in the current understanding of marine
REE cycling is that reversible scavenging controls the vertical
distribution of these elements in the water column. In his deeply
insightful paper, Elderfield (1988) suggested cause for concern
over this conjecture: not for shallow REE adsorption, but in
light of a mechanism for desoprtion at depth. For many trivalent
cations the pH-edge/Langmuir-front for adsorption onto particle
surfaces lies well under a pH of 7 (Morel and Hering, 1993;
Ngwenya et al., 2010); thus, over a typical oceanic pH range
of 7.6 and 8.2 desorption is unlikely. The thermodynamics of
desorption become even less favorable as the increasing dissolved
concentration at depth would tend to push the equilibrium
toward further adsorption (reiterating arguments posed by
Elderfield, 1988). Moreover, given free Nd is <16% of the total
dissolved Nd (Millero, 1992; Schijf et al., 2015), and surface
adsorption coefficients are typically not exceptionally high (in the
range of 3 to 5 onto carboxyl and moncarboxylic groups; Smith
and Martell, 1989; Byrne and Kim, 1990; Ngwenya et al., 2010),
the potential for REE transfer from the shallow to deep ocean
via particle surface scavenging should be limited (Elderfield and
Greaves, 1982; Stichel et al., 2015).
In spite of such arguments, lab and field observations attest
that the REEs are indeed highly susceptible to adsorption
(Sholkovitz, 1989), and that adsorption onto particles in the
ocean certainly does happen (e.g., Sholkovitz et al., 1994;
Tachikawa et al., 1999a). One positive test for REE adsorption
in the oceans may be seen in conservative Arctic Ocean REE
profiles where particles are near absent (Yang and Haley, 2016).
We do note, however, that a counter-argument can be made in
that there does not appear to be an appreciable difference in
the [Nd] profiles relating to marginal sites vs. distal sites, where
particle loading may be very different (German and Elderfield,
1990; Grasse et al., 2012; Goswami et al., 2014; Haley et al., 2014;
Abbott et al., 2015b; Stichel et al., 2015).
Considering all the arguments and data, a preferred model
is that REEs are most likely transferred from the ocean surface
to depth through incorporation onto complex organics, such
as humic acids, polysaccharides, or other complex organic
molecules that have binding coefficients higher than simple
carboxyl groups (logK >9; Byrne and Kim, 1990; Stanley and
Byrne, 1990; Davranche et al., 2005; Pourret et al., 2007; Schijf
et al., 2015). In this way, scavenging is actually achieved via an
organic coating carrier phase that is remineralized, thus offering a
mechanism for transfer of REEs from the particulate to dissolved
phase without subsequent re-adsorption (i.e., not a desorptive
process, but a remineralization; Sholkovitz et al., 1994). This
model implies that [Nd] should behave much like δ13C in the
water column, wherein their distributions reflect a preformed
signal modified by top-down uni-directional input additions
related to POC remineralization. Available GEOTRACES data
allow us to make an initial comparison of water column [Nd]
and δ13C (Mawji et al., 2015; Stichel et al., 2015;Figure 1).
We consider the comparison inconclusive; these data appear to
share many similarities (e.g., at Stations 12, 24, 10, especially
in the upper 1,000 m) but not everywhere (e.g., at Stations
1, 3, and 7). Comparison of these [Nd] and δ13C data yields
no correlation (r2<0.05; Figure S1), although the fit of
all the data from the upper 50 to 500 m, ostensibly under
highest remineralization, is better (r20.4; Figure S2), which
would support scavenging of Nd on isotopically light-C POM.
The form of the [Nd] profiles in Figure 1, especially in the
upper 1,000 m, does appear to share many of the characteristics
of POM remineralization in the oceans (e.g., Suess, 1980;
Martin et al., 1987) and of AOU (discussed by Stichel et al.,
2015). Unfortunately, global compilations, such as presented
by Tachikawa et al. (2017), produce poor correlations between
δ13C and [Nd] (r2<0.001; not shown), and the similarity
between deep (>1,000 m) [Nd] and P(van de Flierdt et al.,
2016) becomes difficult to justify under the hypothesis of POM
scavenging of Nd.
A similar uni-directional release of REEs at depth can also
potentially come from deterioration of the lithogenic minerals
themselves (Rousseau et al., 2015; Abadie et al., 2017). This
mechanism would differ markedly in its effect on water column
[REE] and εNd in comparison to a model based on an organic
surface coating. Unfortunately, the evaluation of [Nd] and εNd in
light of a lithogenic source of Nd at depth has been ambiguous
in the eastern North Atlantic (Stichel et al., 2015; van de Flierdt
et al., 2016).
Evaluation of modes of scavenging may be complicated
through multiple substrates for scavenging, each of which can
deliver REEs to depth (e.g., complex organic molecules as well
as lithogenic and biogenic silicates; Akagi et al., 2011; Akagi,
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Haley et al. Benthic Control Over Marine REEs
10 15 20 25 30 35
Depth (m)
δ13C(permil) δ13C(permil) δ13C(permil)
[Nd](pM) [Nd](pM) [Nd](pM)
Depth (m)Depth (m)
Depth (m)
30˚W 25˚W 20˚W 15˚W 10˚W 5˚W
Ocean Data View
Stns 12
& 24 Stn. 10 Stn. 9
Stn. 7
Stn. 3
Stn. 1
10 15 20 25 30 35
15 20 25 30 35
10 15 20 25 30 35
10 15 20 25 30 35
FIGURE 1 | GEOTRACES [Nd] and δ13 C data from sites in the eastern North Atlantic. Only data where these parameters were available to the sea floor are shown.
Data from Mawji et al. (2015) and Stichel et al. (2015).
2014). Adding additional ambiguity to the assessment of reverse
scavenging is the issue of preformed REEs at depth that are
likely conservative in nature (Haley et al., 2014; Zheng et al.,
2016). Such preformed REEs must be an important component
of deep water REEs (at least on basinal scales), such that REE
profiles cannot be taken at face value: de-convolution of the
conservative and non-conservative REEs is needed for accurate
interpretation (Bertram and Elderfield, 1993; Haley et al., 2014;
Zheng et al., 2016). Such efforts, either using a water mass
approach (e.g., Bertram and Elderfield, 1993; Grasse et al., 2012;
Stichel et al., 2012, 2015 or a more statistical approach (Singh
et al., 2012; Haley et al., 2014; Zheng et al., 2016), are critical
in evaluating the importance of scavenging on water column
REEs. It goes beyond the scope of this paper to explore solutions
for resolving preformed vs. “reactive” REEs. Our goal here is
simply to suggest that much of the difficulty in resolving the “Nd
paradox” is caused by undue emphasis placed on the function
of reverse scavenging, which likely plays only a minor role
in controlling the distribution of REEs and εNd in the water
Our exemplar here is the outstanding work of Bertram and
Elderfield (1993). These authors carefully evaluated the possible
influence of scavenging and reverse scavenging, largely under an
assumption of “top down” forcing, because of the similarities to
Si and a lack of sedimentary data (Bertram and Elderfield, 1993).
Their conclusion is that particle-water exchange is necessary to
explain deep ocean εNd, and give two possible mechanisms: a
“top down” possibility wherein scavenged Nd exchanges over
time in the deep ocean and a “. . . second option [that] simply
places the site of exchange close to or within the sediment-
porewater system with exchange within the nepheloid zone or
transport via diffusion into overlying seawater” (Bertram and
Elderfield, 1993). [Interestingly, the authors also note that “the
simple (water density) & Si-REE correlations seen in bottom and
near-bottom waters may also imply involvement of a benthic
source. Unfortunately, no satisfactory data are available on REEs
in oxic porewaters.”].
Finally, regardless of the impact on the water column, we
emphasize that all of the potential “top down” mechanisms for
remobilizing surface REEs at depth would be greatly enhanced
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Haley et al. Benthic Control Over Marine REEs
within seafloor sediments (Freslon et al., 2014); i.e., manifest
as a pore water flux. In summary, while scavenging certainly
happens on particles, we argue that the REE mass transfer is
likely relatively small and subsequent desorption at depth in the
water column is unlikely. On the other hand, any such desorption
or remineralization mechanism is likely to be significant in
the sediments, where the chemical environment can change
Regardless of the discussions presented above, mechanisms
driven via a “top-down” flux of REEs return to the disparity
between εNd and [Nd] as discussed by Goldstein and Hemming
(2003). The question is fundamentally how can such processes
occur that account for [Nd] distributions, yet does not obviate
the conservative qualities of εNd? That a water mass such as
North Atlantic Deep Water (NADW) can maintain its signature
on ocean-basin scales is arguably less justifiable in a “top-
down” explanation of marine REEs than it is in the “bottom-up”
explanation that will be discussed below.
If we consider our prior argument that reversible scavenging
does not significantly impact Nd’s water column distribution,
what then can explain the distribution of [Nd]/εNd at depth?
A “bottom up” hypothesis argues that Nd and εNd are truly
conservative below the permanent thermocline, except when
exposed to a benthic flux. This benthic flux will certainly impact
overlying bottom water, but its influence may also be carried
laterally along isopycnals, for example off the continental slope
(as indicated by Grasse et al., 2012; Singh et al., 2012; Stichel
et al., 2015). The interaction with the sediments is then what
makes εNd “quasi-conservative,” vs. being truly conservative, as
has been suggested previously (Lacan and Jeandel, 2004b; Abbott
et al., 2015a). The two important aspects of this suggestion are
that (1) in the absence of a positive benthic flux, Nd and εNd
will behave conservatively and (2) the ocean biogeochemistry of
Nd and εNd is thus defined by benthic diagenetic processes. That
is, this benthic control model suggests that the REEs and εNd
are controlled by the nature of the benthic flux, which can be a
positive, negative, or neutral term.
The idea of a benthic source for REEs, as described previously
(Abbott et al., 2015a), would certainly help resolve model and
observational inconsistencies of REE in the Pacific Ocean (Jones
et al., 2008; Arsouze et al., 2009), but is such a source necessary
anywhere else-in the Atlantic, Indian, Arctic or Southern Oceans?
The North Atlantic, for example, has been well described in
“top-down” models (Jones et al., 2008), or even through simple
conservative mixing (van de Flierdt et al., 2016). Although we
note here that the Denmark Straits is the ‘birthplace’ of the
very non-conservative boundary exchange hypothesis (Lacan and
Jeandel, 2004a,b, 2005a) and that other models have required
benthic source fluxes in the North Atlantic (Tachikawa et al.,
Why then invoke a benthic model? First, if correct, the benthic
model would provide a mechanism to reconcile the apparent
conflicts in our interpretation of Nd and εNd; and second, the
benthic model makes different predictions about how changes in
εNd and REEs records can be interpreted, when compared to a
reversible scavenging model, as will be discussed later.
The concept of benthic control of [REE]/εNd on Pacific waters
is still a nascent idea (Abbott et al., 2015a, 2016; Du et al.,
2016). Here, we expand on this idea in a general sense. First,
benthic control of [REE]/εNd will directly influence only the
bottom water, such that a water mass detached from the bottom
will maintain its εNd so long as it maintains its temperature
and salinity (i.e., all its conservative properties). Second, benthic
control represents a diffuse boundary flux, which implies that
deep water [REE]/εNd distributions may be more disposed to
being controlled by eddy diffusivity. Because eddy diffusion
coefficients in the vertical are 105-104m2/s (or greater; Ku
and Luo, 1994), over hundreds to thousands of meters per year
(horizontal coefficients are even greater), the impact of a benthic
flux may potentially integrate over significant depths within the
deep ocean. Furthermore, eddy diffusivity is also an order of
magnitude higher on the seafloor relative to the water column
because of the turbulence created by topography (Waterhouse
et al., 2014); therefore, a benthic flux will be dissipated with
great efficiency into the water column. This dissipation is the
rationale for the observation that there is an apparent jump
in REE concentrations between pore water and overlying water
(Abbott et al., 2015b, 2016).
There is little direct observational data regarding the benthic
REE flux to the oceans (Sholkovitz et al., 1989; Haley et al.,
2004; Abbott et al., 2015b), and even less that tracks the εNd of
this flux. However, comparison with radium offers some support
for the hypothesis. Again, the holistic nature of GEOTRACES
sampling has provided rare, directly comparable Ra-REE-εNd
data (Charette et al., 2015; Stichel et al., 2015; USGT10/GA03
section from the Gulf of Cadiz to the Mauritanian coast). All
isotopes of Ra positively correlate with [Nd] in the bottom water,
although we plot only 228Ra, as this isotope is most directly
associated with a benthic source influence (Figure 2A). The
correlation of 228Ra with [Nd] in bottom water is strong (r2=
0.7 including all the data, and by omitting Site 7, r2=0.9). Such
a positive correlation is consistent with both 228Ra and [Nd]
sharing a common benthic source. In contrast, benthic 228Th
does not covary with [Nd], which might otherwise be expected if
the source of deep Nd was delivered via top-down particle-based
Bottom water εNd does not covary with Ra isotopes
(Figure 2B). While such a correlation is not obligatory under
the benthic control model, work with Pacific sediments suggests
that certain components of the sediments may be more reactive
during diagenesis, and such reactivity is also likely element
specific (Abbott et al., 2016; Du et al., 2016). Thus, we might
expect that reaction of these sedimentary elements imposes a co-
variation between the magnitude and isotopic composition of the
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Haley et al. Benthic Control Over Marine REEs
0.2 0.4 0.6 0.8 1.0
R= 0.96 R= 0.86
0.2 0.4 0.6 0.8 1.0
FIGURE 2 | Comparison of bottom water radium and neodymium (data from
Charette et al., 2015; Stichel et al., 2015). (A) shows the positive correlation of
the benthic-derived Ra with [Nd], consistent with a benthic source for both.
(B) shows little correlation of εNd with Ra, although a correlation may not be
expected between these variables. The unfilled datum is arbitrarily isolated:
there is no reason to consider this datum contaminated.
We reiterate that there is undeniable evidence in support
of the idea that seawater εNd traces basin scale modern water
mass circulation, and the benthic control hypothesis suggested
here does not detract or contradict these observations. On the
contrary, we suggest that records of εNd may potentially be used
to actually quantify current velocities, given that the “exposure
time” of a water mass to the bottom (Abbott et al., 2015a)
implies a temporal sensitivity necessary to establish velocity. As
an example, the bottom water data of Lambelet et al. (2016)
for the North Atlantic, can be plotted vs. latitude (Figure 3). A
benthic control based interpretation of these data may be that
there are three diagenetic regimes in the North Atlantic: north
of 55N, between 45 and 55N and south of 40N. We will first
focus on the increasing [Nd] and decreasing εNd of the middle
regime (43-55N). In this region, there is a total change in εNd
of 2 units, of which conservative mixing with LSW can account
for 1 unit (Figure 4; using water properties defined in van de
Flierdt et al. (2016). Applying the simple exposure time model of
Surface Water
INADW (detached
from bottom)
10 20 30 40 50 60 70
Latitude (°N)
FIGURE 3 | Bottom water [Nd] and εNd from Lambelet et al. (2016). Arrows
show interpretation of flow paths of bottom water.
Abbott et al., 2015a; assuming flux of 20 pmol/cm2/yr with εNd of
15, which is reasonable but arbitrarily chosen, and a deep water
[Nd] of 15 pM, which is measured), we might estimate that the
residual 1-unit corresponds to 35 years of benthic flux exposure
on a 1 km thick bottom water layer, occurring over 12latitude.
This combination results in a bottom water current velocity of 0.2
cm/s. This estimate of effective transport rate, despite its obvious
simplifications and assumptions, is not incomparable to the 1.8–2
cm/s estimates derived from CFC and tritum/He3(Smethie et al.,
2000; Steinfeldt and Rhein, 2004). This simple model scenario
also predicts [Nd] of the water mass increases to 23 pM, similar to
that observed (Figure 3). Other scenarios are obviously possible;
for example, a 5 pmol/cm2/yr flux with εNd of 20 offers a similar
40 year exposure estimate, but predicts a final [Nd] of only 17
pM. The only existing pore waters from the Atlantic (Buzzard’s
Bay; Sholkovitz et al., 1989) show pore water [Nd] of >700 pM
at 13 cm depth in the sediment, which compares to fluxes as
large as any seen in the Pacific (up to 30 pmol/cm2/yr; Haley
and Klinkhammer, 2003; Abbott et al., 2015b). Such a flux of
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Haley et al. Benthic Control Over Marine REEs
(b) (c)
27.70 27.75 27.80 27.85 27.90 27.95 28.00
Potential Density (kg/m3)
NWABW @ 61*N
LSW core
NWABW @ 45*N & AABW @ 37*N
AABW (Stichel et al., 2012)
Conservative mixing
FIGURE 4 | Mixing properties of density and εNd . Conservative mixing of εNd
between NWABW and LSW can explain 1-unit of εNd [dashed arrow (a)],
leaving 1-unit unexplained [dashed arrow (b)]. Compare to non-conservative
alteration via benthic fluxes [solid arrow (c); data from Lambelet et al., 2016].
Note the detached INADW maintains its εNd signature as it does potential
density. The highly non-conservative nature of εNd for AABW is illustrated
(green dashed arrow; see text for details).
30 pmol/cm2/yr with an εNd of 25, as implied might be the
case in this region (Lambelet et al., 2016), predicts only 4 years
of exposure for 1εNd unit change, which equates to 1.1 cm/s
velocity, but an increase of [Nd] to just 16 pM. Again, these
calculations are obviously crude, and poorly constrained; we use
them simply to illustrate sensitivity of deep water to benthic
fluxes, and the feasibility of using the concept for quantitative
reconstruction of circulation.
The Lambelet et al. (2016) data show constancy of εNd in the
Northwest Atlantic Bottom Water (NWABW) after it detaches
(or is displaced) from contact with the bottom south of 40N.
At these lower latitudes, Antarctic Bottom Water (AABW) is in
contact with the sediments, and it is this southern-sourced water
mass that appears to be modified during its transit northwards.
While such a pattern can be explained with a benthic control
model for [REE]/εNd, a “top-down” process that clearly alters
bottom water but not the deep water overlying bottom water
seems difficult to reconcile. Moreover, the potential density of
AABW is remarkably stable along its flow path: σ=27.9 kg/m3at
60S (Stichel et al., 2012) vs. σ=27.903 kg/m3at 37N (from
Lambelet et al., 2016 data; Figure 4). This constancy indicates
that little water mass mixing occurs during this bottom water
transit, yet εNd changes significantly: at least one unit in just
the northern hemisphere data of Lambelet et al. (2016). Again,
if we argue that conservation of NADW εNd precludes a top
down driver for AABW εNd change, the logical conclusion is
that benthic alteration impacts bottom water (i.e., AABW) εNd.
Through the same approximations done above (assuming an
arbitrarily chosen 5 pmol/cm2/yr flux with εNd of 15 and
measured 30 pM [Nd] of bottom water), we might estimate that
the 1-unit 1εNd of AABW observed corresponds to 150 years
of exposure. Over 25of latitude, this corresponds to a current
velocity of 0.06 cm/s, implying that AABW has about one third
the velocity as NWABW. (Note that because this calculation
is based on a concentration gradient, the exact positions of
where the slope is calculated can differ, making such estimations
amenable to paleo-reconstructions where end-members may not
be well constrained.) This effective transport rate estimate is,
as was the case for our NADW estimate, lower than the CFC
estimate of 1.2 cm/s for AABW (Haine et al., 1998). We can
predict that the benthic fluxes affecting AABW must be more
complex, because linear extrapolation of the rate of change in εNd
over latitude as shown in Figures 3,4would imply that AABW
has an εNd of 6 at 60S, which it does not (Stichel et al., 2012).
The likely rationale for this difference is that the diagenetic fluxes
along the greater AABW flow path are more nuanced in both
their magnitude and composition than can be expressed by our
simplified approximations.
There are three further points relevant to this discussion.
First, the benthic control hypothesis does not invoke water
mass mixing to explain patterns in εNd such as those shown in
Figures 3,4) That is, there is no obligate change in conservative
properties, such as temperature or salinity, as the bottom water
εNd is modified. Again, mid-depth water masses detached from
the sediment will likely maintain their εNd signatures, or mix
them as they would other conservative tracer properties along
isopycnals, for example.
Second, the North Atlantic appears to have a propensity to
mask the impacts of a benthic flux because ventilation rates are
high, and thus exposure times are low. Instances where benthic
influences may be directly observed in the North Atlantic (e.g.,
Lacan and Jeandel, 2004c) would thus reflect rather extreme
cases of high flux or divergent isotopic composition. In contrast,
AABW in the South Atlantic (and PDW in the Pacific) circulate
rather more slowly and thus would be prone to longer benthic
exposure, resulting in more obvious impacts of a benthic source
Third, the benthic flux mechanism seems to be intimately
connected to authigenic metal oxides in the sediments (Abbott
et al., 2016), which appear to modulate the REE flux and
its εNd signature. Authigenic metal oxides appear to act as a
“capacitor” of REE: i.e., both a source and sink of pore water
and bottom water REE. Analogous to electrical capacitance,
more abundant sedimentary metal oxides will produce greater
Nd fluxes with less temporally variable εNd signatures (Abbott
et al., 2016). While this mechanism is clearly far from resolved,
it offers an explanation for the apparent lack of influence from
the non-radiogenic lithogenic sediments of the central North
Pacific (Jones et al., 1994). Specifically, we predict that there is
no positive benthic flux from these sediments, or that reactive
diagenetic materials are not present in these sediments (Abbott
et al., 2016; Du et al., 2016), or both. We also note that the non-
radiogenic Nd data presented by Jones et al. (1994) reflect the
post-leach lithogenic fraction of the sediments, which may not
play a role at all in defining the benthic flux as defined by our
proposed mechanism.
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Haley et al. Benthic Control Over Marine REEs
Arguably the most important implication of benthic control on
marine [REE]/εNd is how we interpret paleoceanographic records
of water mass change. For these studies, the difference between a
“top-down” or “bottom-up” control on bottom water and on the
marine budget of REEs becomes paramount for two reasons:
Firstly, most interpretations of paleo-data treat εNd as
fundamentally conservative, discounting the possibility of
changes in REE supply that is not the result of water mass
mixing or circulation. In a reversible scavenging model, this
may be a valid assumption if deep-water sensitivity to surface
ocean change is small. However, this supposition then begs
the question of why reversible scavenging controls the modern
deep [REE]/εNd? On the other hand, in a benthic control
scenario the εNd of bottom water may evolve through non-mixing
processes: such as a change in the benthic flux or a change in
the “exposure time” of the bottom water to this flux (Abbott
et al., 2015b). As such, an εNd record interpreted through a
benthic control model may reflect a change in benthic flux or
exposure time, the latter of which may well be the result of
changing circulation patterns or velocities. Despite the ostensible
complications, we argue that interpreting εNd records purely as
a conservative tracer discounts the “pseudo-conservative” nature
of the tracer. Our benthic control model offers an internal cycling
constraint on [Nd] and εNd compared to a reverse scavenging
model. Both models require the ability to reconstruct fluxes
through time to accurately use εNd as a tracer: the reversible
scavenging model requires estimation of vertical particle fluxes
and particulate type over time, whereas the benthic control model
requires estimation of the benthic flux over time. Determining
either of these fluxes will be non-trivial tasks, but may be
Secondly, similarity between surface sediment archival phases
(dispersed oxides, coatings, fish teeth, forams) and bottom
water is often cited as evidence for record fidelity under the
top down model for εNd. In the benthic control model, such
correspondence will result from bottom water that is either
dominated by a strong positive benthic flux, wherein the
preformed bottom water εNd signature is overwhelmed by a pore
water signature supported by the authigenic phases, or from
a negative or net-zero pore water flux, such that pore water
and authigenic phases passively record bottom water signatures.
There is also a range of intermediate conditions where εNd of
the archival phase may differ from bottom water, and where
we would predict there is a significant, but not overwhelming,
positive benthic flux into bottom water (e.g., Du et al., 2016
in the Pacific, or Gutjahr et al., 2008; Huck et al., 2016 in the
Atlantic). The relationship between authigenic phases (used as
archival records) and contemporaneous bottom water is far more
complex in the benthic control model, because the authigenic
phases associated with diagenesis are intimately associated with
the nature of the benthic flux that impact the bottom water
signal (Abbott et al., 2016). Moreover, the concentrations of
sedimentary reactive particulate phases, pore water, and bottom
water differ by orders of magnitude, so we stress the importance
of looking at both εNd and the [REEs].
Our benthic model may give an initially grim outlook
for paleoproxy work, but the situation is probably not at all
intractable: in fact, these challenges are likely to be fully resolvable
with further observations and models. Most pressing is the need
to better determine the mechanisms and sources that define
the Nd sink terms within the sediment column: these processes
are ultimately what will define the proxy records. For example,
Abbott et al. (2015b) did not discuss the deep (10 cm) sink
term seen in pore water [Nd] profiles; if this sink records
contemporaneous pore water εNd, an apparent temporal offset
could be generated in records of εNd from diagenetic Fe-Mn
oxides and other isotope and trace element proxies archived in
the calcite lattice of foraminifera (Piotrowski et al., 2005).
Theory and observations all indicate that some aspect of
bottom water εNd is maintained in authigenic phases, and is
likely identifiable in all sediment (Gutjahr et al., 2007; Wilson
et al., 2013; Blaser et al., 2016; Du et al., 2016). Furthermore,
the additional Nd generated through diagenesis should be far
less variable and readily constrained as “baseline” variations in
sedimentary records, vs. higher-order changes that are derived
from seawater; analogous to Sr isotope records. A compilation of
global surface sediment authigenic-phase εNd vs. bottom water
εNd (Figure 5) illustrates that authigenic (archival) phases do
trend with bottom water globally, but there is a remarkably
uniform 1- εNd unit offset toward authigenic phases being more
radiogenic: an offset equivalent in the Atlantic and the Pacific.
Furthermore, we note that the Pacific data actually have a more
robust correlation compared to the Atlantic data, consistent with
the preceding discussion that benthic fluxes in the Atlantic are
more difficult to identify from bottom water data. Bearing in
mind that the concentration of Nd in the authigenic phases (at
ppm Nd) is nominally six orders of magnitude greater than the
bottom water (at pM Nd), it is difficult to explain these data
without invoking a Nd source from within the sediments that
is globally persistent. Of course, there are many outstanding
questions to answer before better understanding the benthic
flux of REEs and εNd: for example, can a flux be more non-
radiogenic than the sediment host? How do these fluxes change
over time?
In summary, while complex, diagenesis does follow
predictable rules that make it amenable to modeling, which,
in turn, can provide a robust estimation of past changes in the
benthic control on bottom water Nd. In turn, these modeled
boundary conditions can be used to calculate current direction
and velocities as suggested in the modern examples above.
Moreover, such studies may provide further evidence for changes
in pore waters that can relate to changes in ocean redox or
acid-base chemistry. The benthic model adds complexity, but
also potential for the application of [REE]/εNd.
The REEs and εNd are often touted as being excellent tracers
of processes that impact marine geochemical cycles in a much
broader sense. As such, the hypothesis of benthic control on deep
ocean REE cycling may support suggestions for the importance
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Haley et al. Benthic Control Over Marine REEs
-16 -14 -12 -10 -8 -6 -4 -2 0 2
-16 -14 -12 -10 -8 -6 -4 -2 0 2
5x5 boxes 10x10 boxes
Bottom water εNd
Bottom water εNd
Core top authigenic εNd Core top authigenic εNd
FIGURE 5 | Compiled εNd from core top authigenic phases and Bottom water. These data are compiled as averages from all published data within 5and 10boxes
(ranges within these boxes are shown as error bars). This data compilation is available upon request, or find a similar compilation from Tachikawa et al. (2017).
of these diagenetic processes in marine geochemical cycles
in general. For example, the same authigenic Fe/Mn oxides
apparently involved in the marine cycling of [REE]/εNd are also
rich in other trace metals (e.g., Cu, Zn, Ni, Co). These authigenic
metal oxides may be acting in an expansive way as a capacitor for
geochemical fluxes; i.e., as both a source and sink of these trace
All authors listed have made a substantial, direct and intellectual
contribution to the work, and approved it for publication.
This work was supported by NSF grants OCE-1147407 to JM and
BH, OCE-1357529 to BH, and OCE-1715106 to JM. We thank
Drs. V. Goswami and T. van de Flierdt for their comments, and
Catherine Jeandel for editorial handling of this manuscript.
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
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Supplementary resource (1)

... δ 13 C, 14 C), Nd is not fractionated by marine biological cycling, giving rise to its promise as a carbon-cycleindependent ocean circulation tracer (Blaser et al., 2019a). Yet, a fundamental caveat in the application of ε Nd as a reliable oceanographic tracer is that knowledge of the Nd fluxes in and out of the ocean remains incomplete, and while internal cycling must occur, the efficiency of it with different particle types is poorly constrained (Abbott et al., 2015a;Haley et al., 2017;. Numerical models are useful tools for investigating Nd cycling since they can specify the processes that govern the spatiotemporal variability in Nd isotope distributions in the ocean. ...
... Neodymium isotopes have been simulated in a range of different modelling studies testing specific hypotheses relating to Nd fluxes and thermohaline redistribution (Pöppelmeier et al., 2020a;Arsouze et al., 2009;Rempfer et al., 2011;Siddall et al., 2008;Jones et al., 2008;Roberts et al., 2010;Tachikawa et al., 2003;Ayache et al., 2016;Gu et al., 2019;Oka et al., 2021Oka et al., , 2009Pöppelmeier et al., 2022;Ayache et al., 2023;Pasquier et al., 2022;Du et al., 2020). However, recent work suggests that a seafloor-wide benthic flux, resulting from early diagenetic reactions, may dominate the marine Nd cycle (Haley et al., 2017;Abbott, 2019;Abbott et al., 2015aAbbott et al., , b, 2019Du et al., 2016). These observations, alongside an ever-growing body of high-quality and highly resolved measurements of dissolved seawater Nd concentrations ([Nd]) and ε Nd from the GEOTRACES programme (GEOTRACES Intermediate Data Product Group, 2021), present an opportunity to re-evaluate, revise, and explore constraints on the marine Nd cycle. ...
... Additional measurements from the Tasman Sea inferred the likely presence of a benthic source of similar magnitude to that estimated for the North Pacific, which may indicate that regions with dominantly calcareous sediment also contribute a significant benthic source of Nd (Abbott, 2019). Evidence of this previously overlooked benthic sedimentary source of Nd has led to a shifting paradigm that contends that the dominant addition of Nd to the ocean is from a diffuse sedimentary source at depth, rather than surface point sources from rivers and dust and the shallow to intermediate continental margins (Haley et al., 2017). ...
Full-text available
The neodymium (Nd) isotopic composition of seawater is a widely used ocean circulation tracer. However, uncertainty in quantifying the global ocean Nd budget, particularly constraining elusive non-conservative processes, remains a major challenge. A substantial increase in modern seawater Nd measurements from the GEOTRACES programme, coupled with recent hypotheses that a seafloor-wide benthic Nd flux to the ocean may govern global Nd isotope distributions (εNd), presents an opportunity to develop a new scheme specifically designed to test these paradigms. Here, we present the implementation of Nd isotopes (143Nd and 144Nd) into the ocean component of the FAMOUS coupled atmosphere–ocean general circulation model (Nd v1.0), a tool which can be widely used for simulating complex feedbacks between different Earth system processes on decadal to multi-millennial timescales. Using an equilibrium pre-industrial simulation tuned to represent the large-scale Atlantic Ocean circulation, we perform a series of sensitivity tests evaluating the new Nd isotope scheme. We investigate how Nd source and sink and cycling parameters govern global marine εNd distributions and provide an updated compilation of 6048 Nd concentrations and 3278 εNd measurements to assess model performance. Our findings support the notions that reversible scavenging is a key process for enhancing the Atlantic–Pacific basinal εNd gradient and is capable of driving the observed increase in Nd concentration along the global circulation pathway. A benthic flux represents a major source of Nd to the deep ocean. However, model–data disparities in the North Pacific highlight that under a uniform benthic flux, the source of εNd from seafloor sediments is too non-radiogenic in our model to be able to accurately represent seawater measurements. Additionally, model–data mismatch in the northern North Atlantic alludes to the possibility of preferential contributions from “reactive” non-radiogenic detrital sediments. The new Nd isotope scheme forms an excellent tool for exploring global marine Nd cycling and the interplay between climatic and oceanographic conditions under both modern and palaeoceanographic contexts.
... In recently years, however, two issues have emerged as major challenges to the understanding and application of marine TEIs. First, the modern oceanic budgets of many TEIs, such as Fe, Cu, Ni, Zn and Nd, cannot be balanced by known 35 sources at the riverine, atmospheric and hydrothermal interfaces, and increasing evidence suggests fluxes across the sedimentwater interface (SWI) may play important, if not dominant, roles in setting the global oceanic budgets of TEIs (Homoky et al., 2016;Jeandel, 2016;Little et al., 2014;Du et al., 2020;Haley et al., 2017;Elrod et al., 2004;Cameron and Vance, 2014;Little et al., 2020). Second, applying TEIs as proxies to study the geological evolution of the ocean system is hampered by the lack of quantitative knowledge of the sedimentary cycling of TEIs, which may alter or completely erase the original proxy signals 40 preserved in sedimentary archives (Horner et al., 2021;Crusius and Thomson, 2000). ...
... Neodymium is one of the Rare Earth Elements (REE), which are important tracers in chemical oceanography (Elderfield and Greaves, 1982). Its radiogenic isotope composition, expressed as ε Nd = ( 143 / 144 143 / 144 − 1) × 10 4 where CHUR is the chondritic uniform reservoir, has been used to study modern and past ocean circulation, marine and continental weathering (Goldstein and Hemming, 2003;Haley et al., 2017;Lacan and Jeandel, 2005;Frank, 2002). It is designated as a 1015 ...
... One of the greatest challenge facing the study of the modern ocean Nd cycle is that its sedimentary cycle is poorly constrained. Recently studies suggest that a benthic flux from marine sediments, particularly in the deep sea, is likely the dominant source of seawater Nd, far exceeding that of riverine and dust inputs at the ocean surface (Abbott et al., 2015b;Du et al., 2018Du et al., , 2020Haley et al., 2017). Consequently, pore water εNd, subject to diagenetic processes such as marine silicate weathering, can affect the water 1020 column εNd (Abbott et al., 2015a(Abbott et al., , 2016Du et al., 2016). ...
Full-text available
Trace elements and isotopes (TEIs) are important tools in studying ocean biogeochemistry. Understanding their modern ocean budgets and using their sedimentary records to reconstruct paleoceanographic conditions require mechanistic understanding of the diagenesis of TEIs, yet the lack of appropriate modeling tools has limited our ability to perform such 10 research. Here we introduce SedTrace, a modeling framework that can be used to generate reactive-transport code for modeling marine sediment diagenesis and assist model simulation using advanced numerical tools in Julia. SedTrace enables mechanistic TEI modeling by providing flexible tools of pH and speciation modeling, which are essential in studying TEI diagenesis. SedTrace is designed to solve one particular challenge facing the users of diagenetic models: existing models are usually case-specific and not easily adaptable for new problems, such that the user has to choose between modifying published code and 15 writing their own code, both of which demand strong coding skills. To lower this barrier, SedTrace can generate diagenetic models only requiring the user to supply Excel spreadsheets containing necessary model information. The resulting code is clearly structured and readable, and is integrated with Julia's differential equation solving ecosystems, utilizing tools such as automatic differentiation, sparse numerical methods, Newton-Krylov solvers and preconditioner. This allows efficient solution of large systems of stiff diagenetic equations. We demonstrate the capacity of SedTrace using case studies of modeling the 20 diagenesis of pH, radiogenic and stable isotopes of TEIs.
... non-conservative behaviour, which challenge the simplified conservative assumptions of marine εNd (Tachikawa et al., 2017;Haley et al., 2017;Abbott et al., 2015bAbbott et al., , 2019Wang et al., 2021). Furthermore, due to the inherent challenges of directly measuring fluxes that are both temporally and environmentally variable and globally widespread, there remain limited 100 process-based observations, which are necessary to both compliment the seawater measurements and capture fully the marine geochemical cycling of Nd (Homoky et al., 2016). ...
... The schemes for which Nd cycling has been optimised faced a compounding problem for escalating computational demand by applying resource intensive systematic tuning (i.e., running over 100 simulations), using suboptimal space filling design methodologies that may also lead to inefficient exploration of the input space. In addition, 135 some of these models were restricted by previous assumptions regarding the dominating processes involved in the marine Nd cycle, which limited the development of appropriate model boundary conditions -notably here, an assumption that particleseawater interaction is constrained to shallow continental margins (called into question by studies by Abbott et al., 2015b, a;Abbott, 2019;Haley et al., 2017). ...
... In this study, we tackle these issues head-on, defining a new approach for efficiently refining a recent implementation of Nd isotopes in the ocean component of the Fast Met Office and UK universities Simulator (FAMOUS) GCM (Robinson et al., submitted). The scheme revisited and updated Nd sources, sinks and transformation of the tracer in line with increased observations and current findings relating to global marine Nd cycling (Blanchet, 2019;Robinson et al., 2021;Haley et al., 2017;Siddall et al., 2008;Arsouze et al., 2009), and has been validated against global measurements of seawater [Nd] ...
Full-text available
The neodymium (Nd) isotope composition (εNd) of seawater can be used to trace large-scale ocean circulation features. Yet, due to the elusive nature of marine Nd cycling, particularly in discerning non-conservative particle-seawater interactions, there remains considerable uncertainty surrounding a complete description of marine Nd budgets. Here, we present an optimisation of the Nd isotope scheme within the fast coupled atmosphere-ocean general circulation model (FAMOUS), using a statistical emulator to explore the parametric uncertainty and optimal combinations of three key model inputs relating to: (1) the efficiency of reversible scavenging, (2) the magnitude of the seafloor benthic flux, and (3) a riverine source scaling, accounting for release of Nd from river sourced particulate material. Furthermore, a suite of sensitivity tests provide insight on the regional mobilisation and spatial extent (i.e., testing a margin-constrained versus a seafloor-wide benthic flux) of certain reactive sediment components. In the calibrated scheme, the global marine Nd inventory totals 4.27 × 1012 g and has a mean residence time of 727 years. Atlantic Nd isotope distributions are represented well, and the weak sensitivity of North Atlantic Deep Water to highly unradiogenic sedimentary sources implies an abyssal benthic flux is of secondary importance in determining the water mass εNd properties under the modern vigorous circulation condition. On the other hand, Nd isotope distributions in the North Pacific are 3 to 4 εNd-units too unradiogenic compared to water measurements, and our simulations indicate that a spatially uniform flux of bulk sediment εNd does not sufficiently capture the mobile sediment components interacting with seawater. Our results of sensitivity tests suggest that there are distinct regional differences in how modern seawater acquires its εNd signal, in part relating to the complex interplay of Nd addition and water advection.
... The rare earth elements (REEs), also known as the lanthanides, consist of fourteen elements that have similar chemical properties (Elderfield and Greaves, 1982), and are widely used in paleoceanographic studies (e.g., biogeochemical cycling and sedimentary processes) (Elderfield and Greaves, 1982;Haley et al., 2004;Bayon et al., 2011). The use of REEs as a paleoceanographic tracer depends on the understanding of the cycling of marine REEs (Haley et al., 2017;Deng et al., 2022b). A previous study has suggested that oceanic REEs originate from continental sediments through water-driven and wind-driven transport systems (Stichel et al., 2015). ...
... Some researchers observed rapid changes in the Nd isotopic composition of bottom water (e.g., Lambelet et al., 2016), and mobilization of REEs at the sediment-water interface (Abbott et al., 2015). Ocean's REEs is generally assumed to be derived significantly from marine sediments via upward porewater (Abbott et al., 2015;Chen et al., 2015;Deng et al., 2017;Abbott, 2019), termed as a bottom-up model (Haley et al., 2017). ...
... The input rate of elements to the ocean should be balanced by the rate of burial output, whereas the REEs accumulation rate in sediments is several times higher than the input (Arsouze et al., 2009;Schacht et al., 2010). Haley et al. (2017) proposed that the ''bottom-up" model describe the benthic source of REEs from sediment via porewater to balance the oceanic budget. It is generally assumed that the source of benthic REEs flux may be influenced by continental weathering, sediment provenance and redox environment (Abbott, 2019;Abbott et al., 2019). ...
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The rare earth elements (REEs) are a crucial resource frequently used as proxies for reconstructing paleomarine environmental conditions. However, there is limited understanding regarding the marine REEs cycling. In addition, the relationship between benthic diffusion of REEs and seamounts, remain unclear. Here we present the concentration profiles of REEs and trace elements in porewater and seawater from both the seamounts area and abyssal plain. Our results evidenced that the concentration of dissolved porewater REEs, Mn and Al in sediments in proximity of seamount are much higher than those at the abyssal plain. This is attributed to the substantial benthic diffusion of REEs to the ocean. The REEs enrichment processes in porewater at uneven seafloor are based mainly on re-suspension associated with the strong currents and/ or partial dissolution of detrital from seamounts. We suspect this process may occur in other world regions with seamounts and infer that these sediments may account for a significant contributor to the oceanic REEs budget.
... Additional measurements from the Tasman Sea suggest the likely presence of a benthic source of similar magnitude to that inferred for the North Pacific, which may indicate that regions with dominantly calcareous sediment also contribute a significant benthic source of Nd (Abbott, 2019). Evidence of this previously overlooked abyssal benthic 105 sedimentary source of Nd has led to a shifting paradigm that challenges the current 'top down' model of marine Nd cycling to one of a 'bottom up' model (Haley et al., 2017). The bottom up model contends that the dominant addition of Nd to the ocean is from a diffuse sedimentary source at depth, rather than surface point sources from rivers and dust and the shallow continental margins ('top down'). ...
... Simple box models have been employed to investigate, to a first order, the non-conservative effects from a benthic flux (Du et al., 2016(Du et al., , 2018(Du et al., , 2020Haley et al., 2017;Pöppelmeier et al., 2020b), suggesting overprinting of deep water mass εNd is linked to benthic flux exposure time and the difference between the Nd isotope composition of the benthic flux and the bottom water (Abbott et al., 2015a;Du et al., 2018). However, these models lack comprehensive descriptions of both the marine Nd cycle Pöppelmeier et al. (2021) investigated the benthic flux hypothesis in more detail by updating the Nd isotope enabled Bern3D model (Rempfer et al., 2011) to represent recent observations that indicate a Nd flux from bottom waters could occur across the entire seafloor. ...
... The sediment source describes the flux of Nd into seawater entering each model grid cell adjacent to sediment, this source is not restricted to the uppermost surface layers and can be implemented across all vertical and horizonal sediment-water interfaces, as is the case for all experiments described in this study. As such, the sediment source represents (1) boundary exchange, as defined by Lacan and Jeandel (2005b), which describes the flux of Nd from particle-seawater exchange occurring predominantly across the continental margins; (2) submarine groundwater discharge, as suggested by Johannesson and Burdige 345 (2007), which releases Nd to seawater via discharge of fresh groundwater to coastal seas and is mainly limited to the upper 200 m; and (3) a benthic flux, which specifically refers to a transfer of Nd from sediment pore water to seawater resulting from early diagenetic reactions and is not depth limited (Abbott et al., 2015b, a;Haley et al., 2017). ...
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The neodymium (Nd) isotopic composition of seawater is a widely used ocean circulation tracer. However, uncertainty in quantifying the global ocean Nd budget, particularly constraining elusive non-conservative processes, remains a major challenge. A substantial increase in modern seawater Nd measurements from the GEOTRACES programme coupled with recent hypotheses that a seafloor-wide benthic Nd flux to the ocean may govern global Nd isotope distributions (εNd) presents an opportunity to develop a new scheme specifically designed to test these paradigms. Here, we present the implementation of Nd isotopes (143Nd and 144Nd) into the ocean component of the FAMOUS coupled atmosphere-ocean general circulation model (ND v1.0), a tool which can be widely used for simulating complex feedbacks between different Earth system processes on decadal to multi-millennial timescales. Using an equilibrium pre-industrial simulation tuned to represent the largescale Atlantic Ocean circulation, we perform a series of sensitivity tests evaluating the new Nd isotope scheme. We investigate how Nd source/sink and cycling parameters govern global marine εNd distributions, and provide an updated compilation of 6,048 Nd concentration and 3,278 εNd measurements to assess model performance. Our findings support the notions that reversible scavenging is a key process for enhancing the Atlantic-Pacific basinal εNd gradient, and is capable of driving the observed increase in Nd concentration along the global circulation pathway. A benthic flux represents a major source of Nd to the deep ocean. However, model-data disparities in the North Pacific highlight that the source of εNd from seafloor sediment is too unradiogenic in our model with a constant benthic flux. Additionally, model-data mismatch in the northern North Atlantic suggests a missing source of Nd that is much more unradiogenic than the bulk sediment, alluding to the possibility of preferential contributions from ‘reactive’ detrital sediments under a benthic flux driven model of marine Nd cycling. The new Nd isotope scheme forms an excellent tool for exploring global marine Nd cycling and the interplay between climatic and oceanographic conditions under both modern and palaeoceanographic contexts.
... Submarine groundwater discharge could also be a significant contributor to the REE budget of coastal seawater (Johannesson and Burdige, 2007;Kim and Kim, 2014). In addition, the diffusive benthic flux of REEs from pore fluid to bottom water may account for the "missing" flux of REEs in the ocean, as predicted by modeling (Abbott et al., 2015;Haley et al., 2017). ...
... Seasonal changes in seawater REE concentrations and ϵNd have been reported; these changes are generally attributed to the influence of biogeochemical cycling or seasonally variable terrigenous inputs (Crocket et al., 2018;Grenier et al., 2018;Hathorne et al., 2020). The Nd released from pore fluid can also act as a benthic source to bottom water, leading to the alteration of primary seawater eNd value (Abbott et al., 2016;Du et al., 2016;Haley et al., 2017). Therefore, evaluating the applicability of seawater-derived Nd isotopes in paleoceanographic studies is crucial, especially in various marginal seas that are significantly affected by terrigenous inputs. ...
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Dissolved rare earth elements (REEs) and neodymium isotopes (ϵNd) have been jointly used to evaluate water mass mixing and lithogenic inputs in the ocean. As the largest marginal sea of the West Pacific, the South China Sea (SCS) is an ideal region for reconstructing past hydrological changes. However, its REE and ϵNd distributions and underlying controlling mechanisms remain poorly understood. On the basis of four seawater profiles spread across the SCS, this study presents dissolved REE concentrations and ϵNd data under summer condition to better understand the processes that potentially influence changes in these parameters and their marine cycling. The results show high concentrations of REEs and large variations in ϵNd (−6.7 to −2.8) in surface water, likely caused by the dissolution of riverine and marine particles. Comparison with published data from samples taken during the winter of different years in this and previous studies suggests a possible seasonal variability of middle REE enrichment. The SCS deep water shows a narrow ϵNd range from −4.3 to −3.4, confirming the dominant presence of the North Pacific Deep Water in the deep SCS. The intermediate water in the central SCS is characterized by a more negative ϵNd signal (–4.2 to –3.4) than that found in its counterpart in the West Pacific (–3.5 to –2.8), indicating alterations by deep water through three-dimensional overturning circulation from the northern to southern SCS below ~500 m. The contributions of external sources could be quantitatively estimated for the SCS in terms of Nd. The dissolution of particles from the SCS surrounding rivers (0.26–1.3 tons/yr in summer; 5.6–29 tons/yr in winter) and continental margins (2–12 tons/yr in summer; 23–44 tons/yr in winter) may play an important role in providing additional Nd to the SCS surface water.
... In addition, Nd isotope composition was also measured in selected residual detrital fraction (detrital ε Nd ) of the same samples (Fig. 3f) to examine the diagenetic overprinting on the authigenic ε Nd . The fish teeth ε Nd is considered to be more robust and reliable 18 than leachate ε Nd as a proxy for past water mass circulation due to the possibility of partial dissolution of detrital Nd during the leaching of bulk sediments, which can alter the authigenic ε Nd signature [19][20][21] . The agreement between the fish teeth and leachate ε Nd values (Fig. 2a, Supplementary Fig. 3) gives us confidence that the leachate ε Nd is not affected by the partial dissolution of detrital Nd during the leaching. ...
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Global overturning circulation underwent significant changes in the late Miocene, driven by tectonic forcing, and impacted the global climate. Prevailing hypotheses related to the late Miocene deep water circulation (DWC) changes driven by the closure of the Central American Seaways (CAS) and its widespread impact remains untested due to the paucity of suitable records away from the CAS region. Here, we test the hypothesis of the large-scale circulation changes by providing a high-resolution record of DWC since the late Miocene (11.3 to ~2 Ma) from the north-western Indian Ocean. Our investigation reveals a progressive shift from Pacific-dominated DWC before ~9.0 Ma to the onset of a modern-like DWC system in the Indian Ocean comprising of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (~6 Ma) caused by progressive shoaling of the CAS and suggests its widespread impact. Deep ocean circulation plays a crucial role in controlling global climate. What caused on-set of modern like circulation in geological past remains unknown. New research finds constriction of the Central American Seaway caused on-set of modern-like circulation in Indian Ocean since the late-Miocene (~6 Ma).
... However, none of these two samples, in particularly MOZ1-DR17-01, show Nd isotope values lower than εNd = -9.7 and any linear trend from low radiogenic to more radiogenic Nd isotope compositions is visible in the four studied Fe-Mn crusts within the latitudinal range ( Figure 4.10). It would be the same observation in the context of low radiogenic Nd inputs from pore waters (Abbott, 2019;Abbott et al., 2015;Haley et al., 2017;Haley and Klinkhammer, 2003;Schacht et al., 2010). ...
Le canal du Mozambique, localisé à l’est du craton africain et à l’ouest de la marge continentale de Madagascar, abrite d’intenses mélanges de masses d’eau des océans Atlantique et Indien qui soulèvent des interrogations, à savoir si et comment elles ont circulé au cours du temps, en s’adaptant à la géodynamique du canal. L’analyse géochimique (éléments majeurs, traces ; isotopie Nd, Pb) et la datation absolue (isotopie Be) de 33 encroûtements Fe-Mn, répartis depuis le Plateau des Aiguilles au sud jusqu’au bassin des Comores au nord, permet ici d’établir des séries temporelles afin d’étudier l’évolution des courants régionaux jusqu’à 30.7Ma. Depuis 80 000ans, le NADW circule jusqu’à la ride de Jeffrey alors que dans l’archipel des Comores, le NIDW est majoritaire. Cependant, ce modèle océanographique n’a pas toujours fonctionné ainsi. Dès sa mise en place, le NADW a eu une forte influence jusqu’aux Îles Glorieuses. Entre 11.7 et 3.4Ma, un soulèvement bouleversa la géodynamique du canal, entrainant une élévation de la ride de Davie. Le NADW diminua au nord, jusqu’à se retirer du bassin des Comores, au profit de l’augmentation du NIDW. Entre 5.1 et 1.6 Ma, une subsidence, enregistrée au centre du canal, modifia à son tour la bathymétrie et l’océanographie du secteur. La géochimie révèle également des apports élémentaires téthysiens et himalayens dans les courants profonds. Ainsi, grâce à une étude haute résolution des courants profonds du canal du Mozambique ce doctorat complète les modèles océanographiques de l’océan Indien et innove dans les reconstructions géodynamiques grâce à l’utilisation des encroutements Fe-Mn dans la caractérisation de mouvements verticaux.
Determining past changes in ocean circulation on the Antarctic margin is important for understanding the interactions between climate, circulation, and ice sheet retreat. However, the application of neodymium isotopes (εNd), a well-known proxy of ocean circulation, is limited on the Antarctic margin, due to the lack of carbonate preservation and inconsistency in authigenic εNd leached from bulk sediment. Here we assess the use of the εNd proxy along the continental rise of Wilkes Land, by combining analyses of seawater with phases extracted using a 10-second reductive leach in co-located surface sediments. Dissolved seawater εNd values displayed the following water mass signatures; εNd = - 8.8 ± 0.2 (n=1) for Antarctic Surface Water; εNd = - 9.7 ± 0.2 (n=2) for Winter Water; and εNd = - 8.7 ± 0.3 (n=6) for modified Circumpolar Deep Water. The sediment leachate did not reproduce a bottom water Nd signature and yielded a very variable εNd, ranging from -10.4 to -14.4. In contrast, the bulk detrital sediment εNd fell within a narrow range of -13.1 ± 0.5 (n = 29). Examination of elemental ratios and rare earth element (REE) anomalies indicates that the leaching procedure extracts exclusively authigenic phases. The strong relationship found between phosphorus (P) and REE suggests that a P-associated phase is the main REE host-phase leached, as opposed to ferromanganese oxyhydroxides. The leached europium anomalies and εNd indicate that the extracted Nd signature is influenced by two end members, one with a seawater-like εNd (∼ -8) and one more negative than the bulk detritus (εNd ∼ -14.4). Finally, the decoupling between [Nd] and εNd, as well the distinct middle-REE enrichments measured in the leach implies the authigenic εNd is controlled by diagenesis. We thus infer that the Nd signature of the leached phosphates is derived from porewater and influenced by the presence of reactive detrital components in the sediment. Unradiogenic reactive sedimentary phases which easily dissolve into porewaters are likely sourced from the subglacial erosion of ancient metasediments and crustal remnants on the Wilkes Land coast. These new εNd data contribute to the evaluation of the εNd proxy at complex, dynamic and under-studied interfaces such as the East Antarctic margin. However, further work is still needed to develop reliable proxies of past Antarctic water masses.
The global overturning ocean circulation plays a key role in global climate by distributing heat around the Earth and by triggering or amplifying major climate changes. Neodymium (Nd) isotopes are widely used to trace present-day and past ocean circulation changes; however, their value as a circulation tracer has been increasingly challenged by studies that have focused on processes that can modify seawater Nd isotope ratios (for example, seawater interaction with particles, pore water fluxes from sediments). The Southwest Atlantic represents an excellent test bed for investigating the integrity of Nd isotopes as an ocean circulation tracer, as it includes the major Atlantic northern and southern hemisphere-sourced end-member water masses associated with the global overturning ocean circulation, Antarctic Intermediate Water, North Atlantic Deep Water, and Antarctic Bottom Water (AAIW, NADW, and AABW, respectively) and potential regional sources of Nd that could impact that integrity. This study reports Nd isotope data on the GEOTRACES GA02 Southwest Atlantic Meridional Transect, spanning the equator to ∼50°S. Below the pycnocline, substantial non-conservative behavior is observed only in samples dominated by AAIW in the northern portion of the transect (∼25°S to the equator); this appears to reflect addition of Nd from the cratons of South America or Africa from above the pycnocline. This effect is not observed at depths directly below dominated by NADW. Otherwise, Nd isotopes behave as a near-conservative water mass tracer along the transect, with 48% of samples within analytical error of the predicted value from water mass mixing, and 84% within 0.9 εNd-units (∼3 times the analytical error), thus confirming its potential at most depths and locations in the Southwest Atlantic to reconstruct past ocean circulation changes.
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Rare earth elements have generally not been thought to have a biological role. However, recent work has demonstrated that the light REEs (LREEs: La, Ce, Pr, and Nd) are essential for at least some methanotrophs, being co-factors in the XoxF type of methanol dehydrogenase (MDH). We show here that dissolved LREEs were significantly removed in a submerged plume of methane-rich water during the Deepwater Horizon (DWH) well blowout. Furthermore, incubation experiments conducted with naturally methane-enriched waters from hydrocarbon seeps in the vicinity of the DWH wellhead also showed LREE removal concurrent with methane consumption. Metagenomic sequencing of incubation samples revealed that LREE-containing MDHs were present. Our field and laboratory observations provide further insight into the biochemical pathways of methanotrophy during the DWH blowout. Additionally, our results are the first observations of direct biological alteration of REE distributions in oceanic systems. In view of the ubiquity of LREE-containing MDHs in oceanic systems, our results suggest that biological uptake of LREEs is an overlooked aspect of the oceanic geochemistry of this group of elements previously thought to be biologically inactive and an unresolved factor in the flux of methane, a potent greenhouse gas, from the ocean.
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The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer– tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
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Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143Nd/144Nd ratios (εNd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present εNd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene–Miocene transition (OMT; ∼23 Ma). We compare our εNd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. εNd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (εNd≈−15); much lower than the εNd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (εNd≈−10) and South Atlantic (εNd≈−8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (εNd≈−18) strongly influences the εNd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect εNd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.
Over the past twenty five years the Commission on Equilibrium Data of the Analytical Division of the International Union of Pure and Applied Chemistry has been sponsoring a noncritical compilation of metal complex formation constants and related equilibrium constants. This work was extensive in scope and resulted in publication of two large volumes of Stability Constants by the Chemical Society (London). The first volume, edited by L. G. Sillen (for inorganic ligands) and by A. E. Martell (for organic ligands), was published in 1964 and covered the literature through 1962. The second volume, subtitled Supplement No. 1, edited by L. G. Sillen and E. Hogfeldt (for inorganic ligands), and A. E. Martell and R. M. Smith (for organic ligands), was published in 1971 and covered the literature up to 1969. These two large compilations attempted to cover all papers in the field related to metal complex equilibria (heats, entropies, and free energies). Most recently a noncritical compilation of organic ligands by D. D. Perrin (Pergamon Press) extended coverage of the literature through 1973 and a similar volume for inorganic ligands by E. Hogfeldt covered through 1974. Since it was the policy of the Commission during that period to avoid decisions concerning the quality and reliability of the published work, th~ compilation would frequently contain from ten to twenty values for a single equilibrium constant.
Neodymium isotopic compositions (¹⁴³Nd/¹⁴⁴Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years).
Significance Iron is an essential micronutrient for life. However, its scarcity limits algae growth in about one-half of the ocean. Its cycle is therefore linked to the global carbon cycle and climate. We present an iron isotope section from the Southern Ocean. In contrast to the common but oversimplified view, according to which organic matter remineralization is the major pathway releasing dissolved iron below the surface layers, these data reveal other dominant processes at depth, likely abiotic desorption/dissolution from lithogenic particles. This suggests that the iron cycle, and therefore primary production and climate, may be more sensitive than previously thought to continental erosion, dissolved/particle interactions, and deep water upwelling. These processes likely impact other elements in the ocean.
Groundwater and seawater samples were collected from nearshore wells and offshore along the Kona Coast of the Big Island of Hawaii to investigate rare earth element (REE) behavior in local subterranean estuaries. Previous investigations showed that submarine groundwater discharge (SGD) is the predominant flux of terrestrial waters to the coastal ocean along the arid Kona Coast of Hawaii. Groundwater and seawater samples were filtered through 0.45 μm and 0.02 μm pore-size filters to evaluate the importance of colloidal and soluble (i.e., truly dissolved ionic species and/or low molecular weight [LMW] colloids) fractions of the REEs in the local subterranean estuaries. Mixing experiments using groundwater collected immediately down gradient from a wastewater treatment facility (WWTF) proximal to the Kaloko-Hanokohau National Historic Park, and more “pristine” groundwater from a well constructed in a lava tube at Kiholo Bay, were mixed with local seawater to study the effect of solution composition (i.e., pH, salinity) on the concentrations and fractionation behavior of the REEs as groundwater mixes with seawater in Kona Coast subterranean estuaries. The mixed waters were also filtered through 0.45 or 0.02 μm filters to ascertain the behavior of colloidal and soluble fractions of the REEs across the salinity gradient in each mixing experiment. Concentrations of the REEs were statistically identical (two-tailed Student t-test, 95% confidence) between the sequentially filtered sample aliquots, indicating that the REEs occur as dissolved ionic species and/or LMW colloids in Kona Coast groundwaters. The mixing experiments revealed that the REEs are released to solution from suspended particles or colloids when Kona Coast groundwater waters mix with local seawater. The order of release that accompanies increasing pH and salinity follows light REE (LREE) > middle REE (MREE) > heavy REE (HREE). Release of REEs in the mixing experiments is driven by decreases in the free metal ion activity in solution and the concomitant increase in the amount of each REE that occurs in solution as dicarbonato complexes [i.e., Ln(CO3)2⁻] as pH increases across the salinity gradient. Input-normalized REE patterns of Kona Coast groundwater and coastal seawater are nearly identical and relatively flat compared to North Pacific seawater, indicating that SGD is the chief source of these trace elements to the ocean along the Kona Coast. Additionally, REE concentrations of the coastal seawater are between 10 and 50 times higher than previously reported open-ocean seawater values from the North Pacific, further demonstrating the importance of SGD fluxes of REEs to these coastal waters. Taken together, these observations indicate that large-scale removal of REEs, which characterizes the behavior of REEs in the low salinity reaches of many surface estuaries, is not a feature of the subterranean estuary along the Kona Coast. A large positive gadolinium (Gd) anomaly characterizes groundwater from the vicinity of the WWTF. The positive Gd anomaly can be traced to the coastal ocean, providing further evidence of the impact of SGD on the coastal waters. Estimates of the SGD fluxes of the REEs to the coastal ocean along the Kona Coast (i.e., 1.3 – 2.6 mmol Nd day⁻¹) are similar to recent estimates of SGD fluxes of REEs along Florida’s east coast and to Rhode Island Sound, all of which points to the importance of SGD as significant flux of REEs to the coastal ocean.
Land to ocean transfer of material largely controls the chemical composition of seawater and the global element cycles. Oceanic isotopic budgets of chemical species, macro- and micronutrients (e.g. Nd, Sr, Si, Mg, Zn, Mo and Ni) have revealed an imbalance between their sources and sinks. Radiogenic isotope budgets underlined the importance of taking into account continental margins as a source of elements to oceans. They also highlighted that the net land–ocean inputs of chemical species probably result from particle-dissolved exchange processes, named ‘Boundary Exchange’. Yet, locations where ‘Boundary Exchange’ occurs are not clearly identified and reviewed here: discharge of huge amount of freshly weathered particles at the river mouths, submarine weathering of deposited sediments along the margins, submarine groundwater discharges and subterranean estuaries. As a whole, we conclude that all of them might contribute to ‘Boundary Exchange’. Highlighting their specific roles and the processes at play is a key scientific issue for the second half of GEOTRACES. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.
Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal and detrital neodymium isotope data from three intermediate depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seawater neodymium composition in this location. The new foraminiferal records reveal no changes in seawater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the intermediate depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southwards shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced water during intermediate water production in the Atlantic sector of the Southern Ocean. Our intermediate depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.