[Show abstract][Hide abstract] ABSTRACT: Coccolithophore assemblages recovered from Integrated Ocean Drilling Program (IODP) Site U1313 are investigated to reconstruct the palaeoceanographic evolution of a sector of the North Atlantic during Marine Isotope Stage (MIS) 19 (~ 790-760 ka) at orbital, suborbital and millennial time-scales.
[Show abstract][Hide abstract] ABSTRACT: Since the seminal work by Hays et al. (1976), a plethora of studies has demonstrated a correlation between orbital variations and climatic change. However, information on how changes in orbital boundary conditions affected the frequency and amplitude of millennial-scale climate variability is still fragmentary. The Marine Isotope Stage (MIS) 19, an interglacial centred at around 785 ka, provides an opportunity to pursue this question and test the hypothesis that the long-term processes set up the boundary conditions within which the short-term processes operate. Similarly to the current interglacial, MIS 19 is characterised by a minimum of the 400-kyr eccentricity cycle, subdued amplitude of precessional changes, and small amplitude variations in insolation. Here we examine the record of climatic conditions during MIS 19 using high-resolution stable isotope records from benthic and planktonic foraminifera from a sedimentary sequence in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313) in order to assess the stability and duration of this interglacial, and evaluate the climate system's response in the millennial band to known orbitally induced insolation changes. Benthic and planktonic foraminiferal delta O-18 values indicate relatively stable conditions during the peak warmth of MIS 19, but sea-surface and deep-water reconstructions start diverging during the transition towards the glacial MIS 18, when large, cold excursions disrupt the surface waters whereas low amplitude millennial scale fluctuations persist in the deep waters as recorded by the oxygen isotope signal. The glacial inception occurred at similar to 779 ka, in agreement with an increased abundance of tetra-unsaturated alkenones, reflecting the influence of icebergs and associated meltwater pulses and high-latitude waters at the study site. After having combined the new results with previous data from the same site, and using a variety of time series analysis techniques, we evaluate the evolution of millennial climate variability in response to changing orbital boundary conditions during the Early-Middle Pleistocene. Suborbital variability in both surface- and deep-water records is mainly concentrated at a period of -11 kyr and, additionally, at similar to 5.8 and similar to 3.9 kyr in the deep ocean; these periods are equal to harmonics of precession band oscillations. The fact that the response at the similar to 11 kyr period increased over the same interval during which the amplitude of the response to the precessional cycle increased supports the notion that most of the variance in the 11 kyr band in the sedimentary record is nonlinearly transferred from precession band oscillations. Considering that these periodicities are important features in the equatorial and intertropical insolation, these observations are in line with the view that the low-latitude regions play an important role in the response of the climate system to the astronomical forcing. We conclude that the effect of the orbitally induced insolation is of fundamental importance in regulating the timing and amplitude of millennial scale climate variability.
No preview · Article · Jan 2015 · Quaternary Science Reviews
[Show abstract][Hide abstract] ABSTRACT: Paleoclimate and paleoenvironmental studies in the north-eastern Italian Alps have been hampered by the rarity of well-preserved high-altitude deposits and the lack of high-resolution multi-proxy records with adequate chronological control. This paper presents the first complete Late Glacial to Holocene peat succession from the Dolomites (Danta di Cadore, Belluno, Italian Alps). A 7 m core was used to evaluate the potential of the ombrotrophic Val di Ciampo peat deposit (1400 m a.s.l.) as an archive of environmental and climate change. The depth age scale is based upon independent C-14 and Pb-210 dates and combined with peat stratigraphy demonstrates that the peat core covers more than 13,200 cal BP, extending to the end of the last part of the Late Glacial. Bulk density, inorganic matter content, pore water pH, conductivity, Ca/Mg ratios, and Ca, Sr and Ti trends were used to identify changes in trophic conditions through the bog. The boundary between ombrotrophic and minerotrophic conditions occurs at approximately 400 cm below the surface and demonstrates that this core is the longest Eastern Alpine ombrotrophic record yet obtained, corresponding to 7000 cal BP. The high-resolution chemical data of this peat archive improves our understanding of European Alpine Holocene climate variability and the relationship between natural climate fluctuations and anthropogenic climate change during the present interglacial in the Dolomites.
No preview · Article · Sep 2013 · Quaternary International
[Show abstract][Hide abstract] ABSTRACT: Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.
[Show abstract][Hide abstract] ABSTRACT: Ombrotrophic peatlands are a valuable repository of high-quality
climatic signals because their only source of water is precipitation.
For this reason, they constitute an authentic and detailed archive of
information about past and present patterns in climate change and allow
us to infer the impact of natural and human activities causing trace
element contamination. In the province of Belluno (Northeastern Alps,
Italy), bogs of major interest are located in Danta di Cadore,
46°34'16'' N 12°29'58'' E, and Coltrondo, 46°39'28'' N
12°26'59'' E. This study aims to provide a detailed reconstruction
of past climate and environmental conditions by a novel multi-proxy
method characterized by high-resolution geochemical (trace elements,
rare earth elements, Pb isotopes, organic carbon, humification), and
pollen analysis using the Danta di Cadore and Coltrondo peat bogs.
Major and trace elements in the peat profiles were measured using the
non-destructive X-Ray Fluorescence (XRF) core scanner. Moreover,
lightness and colour parameters were measured on the images recorded by
a digital colour line scan camera connected to the XRF core scanner.
This method provides high-resolution geochemical data (0.25 cm) and
sediment reflectance spectra (0.1 cm) that document the down-core
distribution of major and trace elements, the changes in sediment
colour, and help to infer the natural geochemical processes which affect
the peat profiles. Conversion of element intensities measured by XRF
core scanner to element concentrations is essential for quantitative
applications involving mass-balance and flux calculations. We calibrate
the of measured intensities of our XRF results with control specimens
taken from the same core at strata corresponding exactly to core-scanner
measurements. The control specimen compositions are quantified using an
Inductively Coupled Plasma Mass Spectrometer (ICP-MS) which is a
well-established method of geochemical analysis. Our measurements of
specific element concentrations describe the trophic status of the peat
profile and, together with a reliable age model, allow the
reconstruction of the changing rates and the predominant sources of a
wide variety of atmospheric trace elements. To our knowledge, this is
the first attempt to provide quantitative geochemical interpretation of
XRF core scanner data for this type of deposit. The results of this type
of analysis provide environmental information which is currently lacking
in the Northeastern Italian Alps.
[Show abstract][Hide abstract] ABSTRACT: Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.
No preview · Article · Dec 2011 · Paleoceanography
[Show abstract][Hide abstract] ABSTRACT: Records of benthic calcite delta18O and Mg/Ca will be presented for a site in Lower Circumpolar Deep Water in the Pacific sector of the Southern Ocean extending from the MIS 37/38 boundary at the start of the mid Pleistocene Transition. Almost half of the flux of cold bottom water entering the major basins of the world ocean does so through the Southwest Pacific Ocean as the Pacific Deep Western Boundary Current which mainly transports Circumpolar Deep Water. Thus, the site occupies a critical region for comparison with Southern Ocean atmospheric records. It should be more representative of the global deep ocean than sites that may have been influenced by hydrographic variations between ocean basins. There is a strong correlation between the climate from the ice core records and that derived from the deep sea, including warmer then Holocene MIS 5, 7, 9 and 11 (but cooler than Antarctic temperatures by 1sigma in normalized data). The lengths and intensities of interglacials will be discussed. Estimation of polar amplification suggests glacial temperature thresholds close to freezing point, not observed at a North Atlantic site in NADW with which the LDCW site will be compared. The role of carbonate ion saturation will be discussed. Seawater delta18O implies ~ 1.5 ○C cooling preceded ice-sheet build up with rapid change at MIS 22 associated with 100 kyr cyclicity. The temperature records from the South Pacific and North Atlantic Ocean show an apparent phase lead of oceanic temperature with respect to eccentricity that will be discussed.
[Show abstract][Hide abstract] ABSTRACT: The Mid-Pleistocene transition (MPT) was the time when quasi-periodic (∼ 100 kyr), high-amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing, leading to the establishment of the characteristic pattern of late Pleistocene climate variability. It has long been known that the interval around 900 ka stands out as a critical point of the MPT, when major glaciations started occurring most notably in the northern hemisphere. Here we examine the record of climatic conditions during this significant interval, using high-resolution stable isotope records from benthic and planktonic foraminifera from a sediment core in the North Atlantic (Integrated Ocean Drilling Program Expedition 306, Site U1313). We have considered the time interval from late in Marine Isotope Stage (MIS) 23 to MIS 20 (910 to 790 ka). Our data indicate that interglacial MIS 21 was a climatically unstable period and was broken into four interstadial periods, which have been identified and correlated across the North Atlantic region. These extra peaks tend to contradict previous studies that interpreted the MIS 21 variability as consisting essentially of a linear response to cyclical changes in orbital parameters. Cooling events in the surface record during MIS 21 were associated with low benthic carbon isotope excursions, suggesting a coupling between surface temperature changes and the strength of the Atlantic meridional overturning circulation. Time series analysis performed on the whole interval indicates that benthic and planktonic oxygen isotopes have significant concentrations of spectral power centered on periods of 10.7 kyr and 6 kyr, which is in agreement with the second and forth harmonic of precession. The excellent correspondence between the foraminifera δ18O records and insolation variations at the Equator in March and September suggests that a mechanism related to low-latitude precession variations, advected to the high latitudes by tropical convective processes, might have generated such a response. This scenario accounts for the presence of oscillations at frequencies equal to precession harmonics at Site U1313, as well as the occurrence of higher amplitude oscillations between the MIS22/21 transition and most of MIS 21, times of enhanced insolation variability.
No preview · Article · Apr 2010 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: The sensitivity to temperature of Mg/Ca ratios in the shallow-infaunal benthic foraminifera Uvigerina spp. has been assessed. Core-top calibrations over ∼1–20°C show a range in sensitivity of 0.065–0.084mmol/mol/°C but few data are available spanning the temperature range anticipated in deep-sea records over glacial–interglacial cycles. In contrast to epibenthic foraminiferal species, carbonate ion saturation appears not to affect Mg/Ca significantly. A method based on estimating the ratio of the temperature sensitivity of foraminiferal Mg/Ca to that of δ18Ocalcite shows that sensitivity for Mg/Ca at the high end of the observed core-top range (∼0.1mmol/mol/°C) is required for consistency with LGM–Holocene differences in each property as constrained by independent proxy data. This is supported by a Mg/Ca record for Uvigerina spp. generated for the Southern Ocean over the past 440,000 years from Ocean Drilling Program Site 1123 (Chatham Rise, New Zealand). The record shows variability that correlates with climate oscillations. The LGM deep ocean temperature derived from the Mg/Ca record is −1.1±0.3°C. Transformation to temperature allows estimates to be made of changes in bottom water temperature and seawater δ18O and comparison made with literature records. Analysis reveals a ∼2.5-kyr lead in the record of temperature over calcite δ18O and a longer lead over seawater δ18O. This is a reflection of larger phase offsets at eccentricity periods; phase offsets at tilt and precession are within error zero.
[Show abstract][Hide abstract] ABSTRACT: One of the most intriguing aspects of Pleistocene climate is the
development of quasi-periodic (ca.100 kyr), high-amplitude glacial
variability during the middle Pleistocene. A number of causes have been
suggested for this Mid-Pleistocene Transition (MPT), but there is no
consensus yet on this matter despite more than two decades of research.
Most hypotheses invoke either a response to a long-term cooling,
possibly induced by decreasing atmospheric pCO2, or changes in internal
ice-sheet dynamics. Marine records from the North Atlantic and
tropical-ocean upwelling regions have already provided support for the
first hypothesis, documenting decreases of sea-surface temperatures
(SSTs) during the MPT; in contrast, no discernible shift in SSTs has
been identified in the Western Pacific Warm Pool. Little is known about
how deep-water temperatures evolved during this climate transition, how
deep-water temperatures correlated with the surface water variability
and how temperature change is linked with the records of greenhouse
gases and atmospheric temperature from EPICA Dome C. Here we present
records of Mg/Ca and stable isotopes measured in planktonic and benthic
foraminifera over the time interval 0.35-1 Ma (Marine Isotope Stages
10-27) from a marine sediment core recovered in the mid-Southern
latitudes (ODP Site 1123, Southwest Pacific Ocean). We have analysed
three foraminiferal species: Globigerina bulloides, Globorotalia inflata
and Uvigerina spp. to represent the upper sea-surface, the thermocline
and the deep ocean respectively. The Mg/Ca-based temperature estimates
allow us to test the hypothesis of a global cooling associated with the
MPT in the Southern Hemisphere and, paired with the foraminiferal d18O
record, derive the d18O of the water in which the foraminifer calcified,
the latter of which combines a global glacioeustatic signal with local
hydrographic effects. Temperature estimates suggest that the long term
average surface- and deep- water temperatures changed relatively little
in the South-west Pacific through the interval analysed, with no
systematic shift across the MPT. In addition, these results permit
preliminary discussion of the magnitude of surface- and deep-water
temperature changes during glacial/interglacial transitions and the
interglacials themselves. The phase relationship between surface- and
deep-water signals has been assessed; in particular, benthic Mg/Ca and
planktonic Mg/Ca are almost in phase at eccentricity, tilt and
precession, with a slight lead of the planktonics over the benthics. At
the 100 kyr period, Mg/Ca leads d18O both in the planktonic and benthic
records. Finally, the phasing of variations in the marine record will be
considered with respect to other component of the climate system, and a
comparison with greenhouse gases and atmospheric temperatures from EPICA
Dome C will be attempted, after evaluation of methods for precise
synchronization of these records.
[Show abstract][Hide abstract] ABSTRACT: One of the most intriguing aspects of the Pleistocene climate is the development of 100 kyr glacial variability during the middle Pleistocene. Very little is known about how deep-water temperatures evolved during this climate transition, how it correlated to the surface water variability and how it links with the records of greenhouse gases and atmospheric temperature from EPICA Dome C. We present detailed records of Mg/Ca, Sr/Ca, B/Ca, Li/Ca and stable isotopes on the infaunal benthic (Uvigerina spp.) foraminifer over the time interval 0-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). Associated records for planktonic (Globigerina bulloides and Globorotalia inflata) foraminifera have also been obtained. The Mg/Ca record has been assessed for secondary effects additional to temperature and comparisons made with ancillary trace element data. The Mg/Ca-based water temperature estimates allow us to examine the deep ocean response to glacial-interglacial changes in the atmosphere and surface ocean over this critical period and, using the paired foraminiferal O isotope and Mg/Ca records explore the changes in seawater O isotopic compsition.
[Show abstract][Hide abstract] ABSTRACT: One of the most intriguing aspects of the Pleistocene climate is the development of quasi-periodic (ca.100 kyr), high-amplitude glacial variability during the middle Pleistocene. A number of causes have been suggested for this Mid-Pleistocene Transition (MPT), but there is no consensus yet on this matter despite more than two decades of research. Most hypotheses for the origin of the MPT invoke either a response to a long-term cooling, possibly induced by decreasing atmospheric pCO2, or changes in internal ice sheet dynamics. Marine records from the North Atlantic and tropical-ocean upwelling regions have already provided support to the first hypothesis, documenting decreases of sea surface temperatures during the MPT. In contrast, very little is known about how deep-water temperatures evolved during this climate transition, and how it correlated to the surface water variability. Here we present records of Mg/Ca and stable isotopes on planktonic (Globigerina bulloides, Globorotalia inflata) and benthic (Uvigerina spp.) foraminifera over the time interval 0.35-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). The Mg/Ca-based temperature estimates allow us to test the hypothesis of a global cooling associated with the MPT in the Southern Hemisphere and, paired with the foraminiferal delta18O record, derive the delta18O of the water in which the foraminifer calcified, the latter of which combines a global glacioeustatic signal with local hydrographic effects. These results permit preliminary discussion of the magnitude of the surface- and deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. The phase relationship between surface- and deep-water signals is tentatively assessed. Finally, the phasing of variations in the marine record will be considered with respect to other component of the climate system, and a comparison with greenhouse gases and atmospheric temperature from EPICA Dome C will be attempted, after evaluation of methods for precise synchronization of these records.
[Show abstract][Hide abstract] ABSTRACT: ODP Site 1240 from the Eastern Equatorial Pacific (EEP) provides a unique archive to explore, at high resolution, the role of low latitude oceanographic systems in past global climate changes. A combination of both inorganic and organic geochemical proxies is applied to reconstruct temperature and salinity changes in surface and thermocline layers. The Sea Surface Temperature (SST) record based on G. ruber Mg/Ca ratios supports a surface warming during times of rapid cooling in the North Atlantic associated to the Heinrich Events. These results are consistent with speleothems records from Brazil and coupled climate model reconstructions, and support that diminished North Atlantic Meridional Overturning Circulation (AMOC) induced a southward displacement of the Inter-Tropical Convergence Zone (ITZC) and hence a weakening of the south-east trade winds. Nevertheless, a SST multi-proxy comparison illustrates a more complex pattern with enhanced seasonality during the last deglaciation induced by both orbital and AMOC forcings interacting at this time. Thermocline proxies support that combined low precession and high obliquity conditions lead to the establishment of "La Niña-like" conditions in the EEP that alternated with diminished upwelling conditions at annual or interannual time scale which apparently resembled intense "El Niño-like" conditions.
[Show abstract][Hide abstract] ABSTRACT: Past hydrographical conditions in the Eastern Equatorial Pacific (EEP) are analysed throughout the multi- proxy study of the long sediment core ODP Site 1240, located at the Panama Basin (0°01.311"N, 86°27.758"W, 2921mbsl). The combination of different geochemical proxies (delta18O, delta13C and Mg/Ca) in two different foraminifera species with diverse preferential dwelling depths in the water column (G. ruber and N. dutertrei) allows the parallel reconstruction for the last 275.000 years of temperature and salinity both at the surface and thermocline layers respectively. The records obtained cover the last 275,000 years at orbital but also millennial-centennial time scales. Results show the existence of recurrent events characterized by saltier thermocline waters which are associated with an intensification of the EEP upwelling system. These events coincide with Earth's orbital configurations characterized by low precession periods and were particularly strong when co-existed with high obliquity conditions, as consistently occurs at the onset of glacial Terminations. This orbital configuration strengthened the atmospheric Walker circulation in the equatorial Pacific which eventually favours the establishment of strong La Niña-like like conditions in the EEP. The thermocline proxies also support the advection of Southern Ocean waters into the equatorial thermocline during glacial Terminations. This 'tunnelling' effect could have been reinforced by the strong La Niña-like conditions through a southward displacement of the westerlies (as supported by modern satellite observations during La Niña events), therefore enhancing the Antarctic Circumpolar Current and the resumption of the Southern Ocean upwelling system. The Site 1240 records also show significant variability at millennial time-scales and demonstrate that sea surface temperatures (SST) evolved consistently with changes in atmospheric CO2. Apparent contradictory results coming from different SST proxy records based on foraminifera Mg/Ca and biomarker (Uk"37) are interpreted in terms of seasonality intensity changes which provide new insights on the understanding of the regional paleoceanography. Results also support a close linkage between oceanographic changes in the EEP and rapid changes in the intensity of the North Atlantic Meridional Overturning circulation.
[Show abstract][Hide abstract] ABSTRACT: Interannual-decadal variability in the equatorial Pacific El Niño–Southern Oscillation (ENSO) induces climate changes at global scale, but its potential influence during past global climate change is not yet well constrained. New high-resolution eastern equatorial Pacific proxy records of thermocline conditions present new evidence of strong orbital control in ENSO-like variability over the last 275,000 years. Recurrent intervals of saltier thermocline waters are associated with the dominance of La Niña–like conditions during glacial terminations, coinciding with periods of low precession and high obliquity. The parallel dominance of δ13C-depleted waters supports the advection of Antarctic origin waters toward the tropical thermocline. This “oceanic tunneling” is proposed to have reinforced orbitally induced changes in ENSO-like variability, composing a complex high- and low-latitude feedback during glacial terminations.