The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

Abstract

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.

Full text

HYPOTHESIS AND THEORY
published: 21 December 2017
doi: 10.3389/fmars.2017.00426
Frontiers in Marine Science | www.frontiersin.org 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
*Correspondence:
Brian A. Haley
bhaley@coas.oregonstate.edu
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
Citation:
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
INTRODUCTION
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
isotopes.
MODERN OCEAN REES AND Nd
ISOTOPES
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
cycling.
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 (r2∼0.4; 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
-0.500.511.52
10 15 20 25 30 35
Depth (m)
[Nd](pM)
δ13C(permil)
δ13C(permil) δ13C(permil) δ13C(permil)
[Nd](pM) [Nd](pM) [Nd](pM)
1000
2000
3000
4000
5000
0
Depth (m)Depth (m)
Depth (m)
15˚N
20˚N
25˚N
30˚N
35˚N
40˚N
30˚W 25˚W 20˚W 15˚W 10˚W 5˚W 0˚
Ocean Data View
Stns 12
& 24 Stn. 10 Stn. 9
Stn. 7
Stn. 3
Stn. 1
0
1000
2000
3000
4000
5000
-0.500.511.52
10 15 20 25 30 35
-0.500.511.52
15 20 25 30 35
0
1000
2000
3000
4000
5000
10 15 20 25 30 35
0
1000
2000
3000
4000
5000
-0.500.511.52
-0.500.511.52
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
column.
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
significantly.
BENTHIC CONTROL
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.,
2003).
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 10−5-10−4m2/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).
IMPLICATIONS OF A BENTHIC FLUX IN
THE ATLANTIC
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
mechanisms.
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
flux.
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Haley et al. Benthic Control Over Marine REEs
18
20
22
24
26
28
30
0.2 0.4 0.6 0.8 1.0
R= 0.96 R= 0.86
[Nd](pM)
228Ra
228Ra
-12.5
-12.0
-11.5
-11.0
-10.5
-10.0
0.2 0.4 0.6 0.8 1.0
εNd
A
B
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 55◦N, between 45 and 55◦N and south of 40◦N. We will first
focus on the increasing [Nd] and decreasing εNd of the middle
regime (∼43◦-55◦N). 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
NWABW
AABW
Surface Water
INADW (detached
from bottom)
-18
-16
-14
-12
-10
-8
10 20 30 40 50 60 70
10
15
20
25
30
35
[Nd](pM)
εNd
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 ∼12◦latitude.
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
(a)
(b) (c)
27.70 27.75 27.80 27.85 27.90 27.95 28.00
-14
-13
-12
-11
-10
-9
εNd
Potential Density (kg/m3)
NWABW
AABW
INADW (NWABW)
LSW
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 ∼40◦N.
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
∼60◦S (Stichel et al., 2012) vs. σ=27.903 kg/m3at ∼37◦N (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 25◦of 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 60◦S, 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
flux.
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
PALEOPROXY Nd
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
resolvable.
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.
BEYOND THE REES AND 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
-16 -14 -12 -10 -8 -6 -4 -2 0 2
-16
-14
-12
-10
-8
-6
-4
-2
0
2
Pacific
Atlantic
Indian
Arctic
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 5◦and 10◦boxes
(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
metals.
AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct and intellectual
contribution to the work, and approved it for publication.
ACKNOWLEDGMENTS
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.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fmars.
2017.00426/full#supplementary-material
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