230 research outputs found
Zinc and cadmium in benthic foraminifera as tracers of ocean paleochemistry
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2000Benthic foraminiferal δ13C, Cd/Ca, and Ba/Ca are important tools for reconstructing
nutrient distributions, and thus ocean circulation, on glacial-interglacial timescales.
However, each tracer has its own "artifacts" that can complicate paleoceanographic
interpretations. It is therefore advantageous to measure multiple nutrient proxies with the
aim of separating the various complicating effects. Zn/Ca is introduced as an important
aid toward this goal.
Benthic (Hoeglundina elegans) Cd/Ca ratios from the Bahama Banks indicate that the
North Atlantic subtropical gyre was greatly depleted in nutrients during the last glacial
maximum (LGM). A high-resolution Cd/Ca record from 965 m water depth suggests that
Glacial North Atlantic Intermediate Water formation was strong during the LGM,
weakened during the deglaciation, and strengthened again during the Younger Dryas cold
period. Comparison of Cd/Ca and δ13C data reveals apparent short-term changes in
carbon isotopic air-sea signatures.
Benthic foraminiferal Zn/Ca could be a sensitive paleoceanographic tracer because deep
water masses have characteristic Zn concentrations that increase about ten-fold from the
deep North Atlantic to the deep North Pacific. A "core top calibration" shows that Zn/Ca
is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and BalCa, by
bottom water saturation state with respect to calcite Since Zn/Ca responds to a different
range of saturation states than Cd/Ca, the two may be used together to evaluate changes
in deep water carbonate ion (CO32-) concentration.
Zn/Ca and Cd/Ca ratios in the benthic foraminifer Cibicidoides wuellerstorfi exhibit large
fluctuations over the past 100,000 years in a deep (3851 m) eastern equatorial Pacific
sediment core. The data imply that bottom water CO32- concentrations were lowest during
glacial Marine Isotope Stage 4 and highest during the last deglaciation. LGM CO32- concentrations
appear to have been within a few μmol kg-1 of modern values.
Deep North Atlantic Cd/Ca ratios imply much higher nutrient concentrations during the
LGM. Although such data have usually been explained by a northward penetration of
Southern Ocean Water (SOW), it has been suggested that they could result from
increased preformed nutrient levels in the high-latitude North Atlantic or by increased
aging of lower North Atlantic Deep Water (NADW). Glacial Zn/Ca data, however,
require a substantially increased mixing with SOW and thus a reduction in NADW
formation. Large changes in carbon isotopic air-sea exchange are invoked to reconcile
benthic δ13C and trace metal data.This work was supported by a JOIlUSSAC Ocean Drilling Fellowship (subgrant
JSG-CY 12-4), the R. H. Cole Ocean Ventures Fund, the Joint Program Education Office,
and the National Science Foundation (grants OCE-9402804 and OCE-9503135 to W.
Curry, and grant OCE-9633499 to D. Oppo)
Oceanic heat advection to the Arctic in the last Millennium
EGU2011-8738
At present, the Arctic is responding faster to global warming than most other areas on earth, as indicated by rising air temperatures, melting glaciers and ice sheets and a decline of the sea ice cover. As part of the meridional overturning circulation which connects all ocean basins and influences global climate, northward flowing Atlantic Water is the major means of heat and salt advection towards the Arctic where it strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ca. 150 years. To reconstruct the history of temperature variations in the Fram Strait Branch of the Atlantic Current we analyzed a marine sediment core from the western Svalbard margin. In multidecadal resolution the Atlantic Water temperature record derived from planktic foraminifer associations and Mg/Ca measurements shows variations corresponding to the well-known climatic periods of the last millennium (Medieval Climate Anomaly, Little Ice Age, Modern/Industrial Period). We find that prior to the beginning of atmospheric CO2 rise at ca. 1850 A.D. average summer temperatures in the uppermost Atlantic Water entering the Arctic Ocean were in the range of 3-4.5°C. Within the 20th century, however, temperatures rose by ca. 2°C and eventually reached the modern level of ca. 6°C. Such values are unprecedented in the 1000 years before and are presumably linked to the Arctic Amplification of global warming. Taking into account the ongoing rise of global temperatures, further warming of inflowing Atlantic Water is expected to have a profound influence on sea ice and air temperatures in the Arctic
Central Equatorial Pacific Cooling During the Last Glacial Maximum
Establishing tropical sea surface temperature (SST) during the Last Glacial Maximum (LGM) is important for constraining equilibrium climate sensitivity to radiative forcing. Until now, there has been little data from the central equatorial Pacific in global compilations, with foraminiferal assemblage‐based estimates suggesting the region was within 1°C of modern temperatures during the LGM. This is in stark contrast to multi‐proxy evidence from the eastern and western Pacific and model simulations which support larger cooling. Here we present the first estimates of glacial SST in the central equatorial Pacific from Mg/Ca in Globigerinoides ruber. Our results show that the central Pacific cooled by about 2.0°C during the LGM, in contrast with previous global compilations but in agreement with models. Our data support a larger magnitude of tropical LGM cooling, and thus a larger equilibrium climate sensitivity, than previous studies which relied on foraminiferal assemblages in the central tropical Pacific
Seawater cadmium in the Florida Straits over the Holocene and implications for Upper AMOC variability
Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 37, (2022): e2021PA004379, https://doi.org/10.1029/2021pa004379.Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer-term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient-like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle.This work was funded by the NSF Graduate Research Fellowship DGE-1148903 (SV) and NSF grant OCE-1459563 and OCE-1851900 (JLS)
Data constraints on glacial Atlantic Water mass geometry and properties
© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Paleoceanography and Paleoclimatology 33 (2018): 1013-1034, doi:10.1029/2018PA003408.The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.National Science Foundation (NSF) Grant Numbers: OCE‐0750880, OCE‐1335191, OCE‐1558341, OCE‐1536380;
Woods Hole Oceanographic Institution (WHOI) Grant Numbers: 27007592, 2700080
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The remote response of the South Asian Monsoon to reduced dust emissions and Sahara greening during the middle Holocene
Previous studies based on multiple paleoclimate archives suggested a prominent intensification of the South Asian Monsoon (SAM) during the mid-Holocene (MH, similar to 6000 years before present). The main forcing that contributed to this intensification is related to changes in the Earth's orbital parameters. Nonetheless, other key factors likely played important roles, including remote changes in vegetation cover and airborne dust emission. In particular, northern Africa also experienced much wetter conditions and a more mesic landscape than today during the MH (the so-called African Humid Period), leading to a large decrease in airborne dust globally. However, most modeling studies investigating the SAM changes during the Holocene overlooked the potential impacts of the vegetation and dust emission changes that took place over northern Africa. Here, we use a set of simulations for the MH climate, in which vegetation over the Sahara and reduced dust concentrations are considered. Our results show that SAM rainfall is strongly affected by Saharan vegetation and dust concentrations, with a large increase in particular over northwestern India and a lengthening of the monsoon season. We propose that this re- mote influence is mediated by anomalies in Indian Ocean sea surface temperatures and may have shaped the evolution of the SAM during the termination of the African Humid Period
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Multiproxy Reduced-Dimension Reconstruction of Pliocene Equatorial Pacific Sea Surface Temperatures
A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperature (SST) distribution resembling that during El Niño events, which is largely inferred from sea surface temperatures reconstructed from several sites in the equatorial Pacific. We utilize a reduced‐dimension methodology on a compilation of previously published multiproxy (Mg/Ca, Uk′37, TEX86, and foraminifer assemblages) Pliocene SST records from the equatorial Pacific to reconstruct spatial and temporal snapshots of SST anomalies and a time series of Niño indices from 5 to 1 Ma. The use of multiple proxies increases the number of study sites and thereby improves the robustness of the reconstruction. We find that the early Pliocene equatorial Pacific was characterized by a reduced zonal SST difference due to minimal change in the west and extreme warmth in the east which peaked at 4.3 Ma. The intensity of this mean El Niño‐like SST state then gradually diminished toward modern conditions. We also use the Pliocene Niño 4 time series to estimate the past strength of Indian Summer Monsoon given the modern correlation of it to the Niño 4 index. Results indicate the monsoon was weaker throughout the study interval with weakest conditions (~37% less rainfall than modern) occurring at 4.3 Ma, congruent with regional proxy records. In summation, this reduced‐dimension approach spatially and temporally resolves the warm mean state of the Pliocene equatorial Pacific and has numerous applications to inferences of paleoclimate conditions in distal regions teleconnected to El Niño today. Plain Language Summary The Pliocene Epoch (5.3–2.6 million years ago) is the most recent time interval in Earth history when atmospheric carbon dioxide concentrations may have been similar to today and the continents were in their current configuration. For these reasons, Pliocene paleoclimate reconstructions are considered to be a useful indicator of conditions expected by the end of the 21st century. Several marine‐derived Pliocene reconstructions from the equatorial Pacific suggest a sea surface temperature (SST) distribution that resembles SSTs during El Niño events today. El Niño events have widespread impacts including reduced marine productivity in the eastern equatorial Pacific and weakened Indian Summer Monsoon. Insights into Pliocene El Niño‐like SSTs are typically based on paleoclimate reconstructions from a few sites that are then used to infer regional conditions. In this study, we apply a statistical method to a compilation of nine Pliocene SST records across the equatorial Pacific to fill in the spatial gaps in the paleotemperature reconstructions. Our maps of reconstructed Pliocene SSTs reveal that the eastern equatorial Pacific was 3–6 °C warmer than today, which is consistent with a mean El Niño‐like state. Given the modern El Niño‐Indian Summer Monsoon relationship, we estimate that Pliocene monsoon was ~20–40% weaker than today. Key Points Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with largest anomalies in the east A mean El Niño‐like state, characterized by a reduced zonal sea surface temperature difference, existed in the Pliocene equatorial Pacific Pliocene Indian Summer Monsoon is estimated to have been ~20–40% weaker than modern</p
Deep ocean storage of heat and CO2 in the Fram Strait, Arctic Ocean during the last glacial period
MME is funded by the Research Council of Norway and the Co-funding of Regional, National, and International Programmes (COFUND) Marie Sklodowska-Curie Actions under the EU Seventh Framework Programme (FP7), project number 274429, and the Research Council of Norway through its Centres of Excellence funding scheme, grant number 223259.The Fram Strait is the only deep gateway between the Arctic Ocean and the Nordic Seas and thus is a key area to study past changes in ocean circulation and the marine carbon cycle. Here, we study deep ocean temperature, δ18O, carbonate chemistry (i.e., carbonate ion concentration, [CO32-]), and nutrient content in the Fram Strait during the late glacial (35,000-19,000 years BP) and the Holocene based on benthic foraminiferal geochemistry and carbon cycle modelling. Our results indicate a thickening of Atlantic water penetrating into the northern Nordic Seas, forming a subsurface Atlantic intermediate water layer reaching to at least ~2600 m water depth during most of the late glacial period. The recirculating Atlantic layer was characterized by relatively high [CO32-] and low δ13C during the late glacial, and provides evidence for a Nordic Seas source to the glacial North Atlantic intermediate water flowing at 2000-3000 m water depth, most likely via the Denmark Strait. In addition, we discuss evidence for enhanced terrestrial carbon input to the Nordic Seas at ~23.5 ka. Comparing our δ13C and qualitative [CO32-] records with results of carbon cycle box modelling suggests that the total terrestrial CO2 release during this carbon input event was low, slow, or directly to the atmosphere.Publisher PDFPeer reviewe
The Geological Record of Ocean Acidification
Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO2, global warming, and ocean acidification over the past ~300 million years of Earth's history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry—a consequence of the unprecedented rapidity of CO2 release currently taking place
Reversed flow of Atlantic deep water during the Last Glacial Maximum
The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies1, 2 and recent model simulations3 indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic 231Pa/230Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength4, 5, 6. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in 231Pa/230Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of 231Pa/230Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC—with a prominent southerly flow of deep waters originating in the North Atlantic—arose only during the Holocene epoch
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