Reconstruction of east–west deep water exchange in the low latitude Atlantic Ocean over the past 25,000 years (original) (raw)
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Earth and Planetary Science Letters, 1985
Detailed Cd/Ca and (~13C data have been obtained for benthic foraminifera from western North Atlantic and Equatorial Pacific sediment cores. In the modern ocean, both tracers are closely linked to nutrient distributions. The sedimentary records for both tracers indicate that bottom waters overlying the Atlantic site have been nutrient-depleted relative to those at the Pacific site over the last 215,000 years. From this evidence it is reasonable to infer that there has been a continuous net flux of nutrient-depleted water from the western North Atlantic into the Pacific. This exchange has undergone significant fluctuations, with the export of nutrient-depleted Atlantic water diminishing by about a factor of two relative to the inflow from Southern Ocean sources. Over the last 215,000 years, carbon isotope fluctuations in both regions are dominated by variable storage of isotopically light carbon on continents with a lesser contribution from these deep ocean circulation changes. The cadmium signal in the North Atlantic is dominated by changes in deep ocean circulation patterns; cadmium shows less variability in the Pacific which may reflect changes in the global average cadmium content or minor changes in deep Pacific circulation patterns. Using these records to estimate global averages, it appears that glacial ocean water had 22% more Cd and 0.46%0 less 13C than the modern ocean. These numbers are estimates which may be revised as more data become available, although they are not likely to be as much as 20% in error for Cd or 0.29;~ for 13C. Relative North Atlantic Deep Water (NADW) formation rates are modulated with a significant 41 kyr periodicity linked to obliquity-induced variations in high latitude insolation; NADW lags 8 _+ 2 kyr behind insolation, however.
Paleoceanography , 2017
Enhanced vertical gradients in benthic foraminiferal δ 13 C and δ 18 O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North Atlantic sourced interior waters; nonetheless, our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high-resolution depth transects of Cibicidoides spp. δ 13 C and δ 18 O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between~1.0 and 2.5 km was more than 1‰ enriched in δ 13 C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ 18 O was~0.5‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a "deep gateway" effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east-west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.
Paleoceanography, 1998
Eastern tropical Atlantic benthic foraminiferal Ba/Ca and CcL/Ca data from core V30-49 (3093 m) reveal large inferred changes in nutrient concentrations of deep Atlantic waters during the last 250 kyr. Relative changes in North Atlantic Deep Water contribution to this site are estimated by scaling the V30-49 Ccl/Ca record to values of modern end-member water masses; these estimates agree well with the relative structure and timing of circulation changes in the eastern tropical Atlantic reconstructed from a •513C record-based mixing model [Raytoo et al., 1997]. Temporal differences between V30-49 Cd/Ca and Ba/Ca records suggest that the Ba/Ca record reflects changes in circulation with an additional increase in the Ba composition of deep Atlantic water masses during glacial episodes, possibly resulting from increased productivity. Similarity between the •513C and Ba/Ca records suggests that carbon isotopes in the deep glacial Atlantic also reflect productivity increases. 1. Introduction Documenting glacial-interglacial changes in Atlantic deep water chemistry provides constraints on reconstructing deep water circulation changes, a parameter important in evaluating the role of the oceans in global climate change. Defining changes in the production of deep water is important because deep ocean circulation plays a critical role in the redistribution of heat and nutrients. In particular, changes in the formation of North Atlantic Deep Water (NADW) may be a direct link between coupled climate change in the northern and southern hemispheres [Broecker and Denton, 1989
Earth and Planetary Science Letters, 2008
A high-resolution authigenic Nd isotope record has been extracted from the Fe-Mn oxyhydroxide fraction of drift sediments along the Blake Ridge in the North Atlantic. These sediments facilitate reconstruction of the timing and extent of major hydrographic changes in the western North Atlantic since the Last Glacial Maximum (LGM). This is one of the few locations where sediments were deposited in the major flow path of the Western Boundary Undercurrent (WBUC), which transports North Atlantic Deep Water (NADW) southward at the present day. The hydrodynamic setting, however, also causes problems. Authigenic Nd isotope compositions similar to the typical present-day NADW ε Nd value of-13.5 ± 0.5 were only extracted from sediments located within the main water body of the WBUC coinciding with the highest along slope current velocity below 3200 m water depth. Above this depth the authigenic Nd isotopic composition is more radiogenic than measured in a nearby seawater profile and appears to be influenced by downslope and lateral sediment redistribution. Our data suggest that these radiogenic signals were formed at shallow depths in Florida current waters, compromising the recorded ambient deep water Nd isotope signal in the Blake Ridge Fe-Mn oxyhydroxide coatings from intermediate depths during the Holocene and the deglaciation. The unradiogenic Nd isotopic composition typical of present-day NADW is not detectable along the Blake Ridge for any water depth during the LGM. Unlike the deglacial and Holocene sections, the intermediate core from 1790 m water depth did not experience significant sediment focussing during the LGM, in accord with the higher current velocities at this depth, suggesting that at this site an ambient LGM bottom water Nd isotope signal was recorded. Assuming this to be correct, our results indicate that the ε Nd of the shallower glacial equivalent of NADW, the Glacial North Atlantic Intermediate Water (GNAIW) may have been as radiogenic as-9.7 ± 0.4. Since the authigenic Nd isotope compositions of the Holocene and the deglacial sections of the intermediate depth sediment core were biased towards a shallow water signal, this first determination of a GNAIW ε Nd for the LGM will have to be corroborated by results from other locations and archives.
Reversed flow of Atlantic deep water during the Last Glacial Maximum
Nature, 2010
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 proxies 1,2 and recent model simulations 3 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 231 Pa/ 230 Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength 4-6 . 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 231 Pa/ 230 Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of 231 Pa/ 230 Th 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.
The last deglaciation was characterised by a series of millennial scale climate events that have been linked to deep ocean variability. While often implied in interpretations, few direct constraints exist on circulation changes at mid-depths. Here we provide new constraints on the variability of deglacial mid-depth circulation using combined radiocarbon and neodymium isotopes in 24 North Atlantic deep-sea corals. Their aragonite skeletons have been dated by uranium-series, providing absolute ages and the resolution to record centennial scale changes, while transects spanning the lifetime of a single coral allow sub-centennial tracer reconstruction. Our results reveal that rapid fluctuations of water mass sourcing and radiocarbon affected the mid-depth water column (1.7-2.5 km) on timescales of less than 100 years during the latter half of Heinrich Stadial 1. The neodymium isotopic variability (−14.5 to −11.0) ranges from the composition of the modern northern-sourced waters towards more radiogenic compositions that suggest the presence of a greater southern-sourced component at some times. However, in detail, simple two-component mixing between well-ventilated northern-sourced and radiocarbon-depleted southern-sourced water masses cannot explain all our data. Instead, corals from ~15.0 ka and ~15.8 ka may record variability between southern-sourced intermediate waters and radiocarbon-depleted northern-sourced waters, unless there was a major shift in the neodymium isotopic composition of the northern endmember. In order to explain the rapid shift towards the most depleted radiocarbon values at ~15.4 ka, we suggest a different mixing scenario involving either radiocarbon-depleted deep water from the Greenland-Iceland-Norwegian Seas or a southern-sourced deep water mass. Since these mid-depth changes preceded the Bolling-Allerod warming, and were apparently unaccompanied by changes in the deep Atlantic, they may indicate an important role for the intermediate ocean in the early deglacial climate evolution.
Quaternary Science Reviews, 2002
We review the various methods which have been applied to estimate the change of seawater d 18 O (dw) between the Last Glacial Maximum (LGM) and the Holocene. The most accurate constraints on these estimates are provided by the measurement of pore waters d 18 O and by high resolution records of benthic foraminifer d 18 O in the high latitude oceans of both hemispheres. They show that the d 18 O of seawater in the deep ocean during the LGM was 1.0570.20% heavier than today, with significant regional variations. Constraints resulting from ice sheet models are less accurate, because both the volume and isotopic composition of each ice sheet are still poorly known. The amplitude of the benthic d 18 O change between the LGM and the Holocene, together with the d 18 O and d 13 C values of the benthic foraminifera genus Cibicides during the LGM, show that the Southern Ocean deep waters were extremely cold, close to the freezing point. During this time, deep waters of the South Atlantic and the Pacific oceans were at least 1.31C warmer than those of the Southern Ocean. Overall, the glacial deep ocean, below 2500 m, was characterized by extremely cold temperatures, everywhere lower than 01C. d 18 O values of benthic foraminifer from the North Atlantic are highly variable. This variability suggests that deep Atlantic waters were not homogeneous, probably because they resulted from the sinking of different surface water masses at various locations during winter. The deep waters in the North Atlantic were at most 21C warmer than in Southern Ocean. Alternatively, they could have been nearer the freezing point with a d 18 O value lighter than the mean ocean water. Brine formation during winter would preserve such light d 18 O values of the northern North Atlantic surface water.
Earth and Planetary Science Letters, 2008
δ 13 C benthic foraminiferal B/Ca ocean circulation North Atlantic Seawater carbonate ion and δ 13 C are affected by many processes including biology, air-sea exchange, alkalinity change, and mixing between different water masses. Study of modern ocean data shows that deep ocean carbonate ion and δ 13 C of dissolved inorganic carbon can be used together as useful tracers for deep water mass reconstructions in the past. We present records of deep water carbonate ion concentration ([CO 3 2− ]) changes of the North Atlantic Ocean water column since the last glacial, quantitatively reconstructed using benthic foraminiferal boron/calcium (B/Ca) ratios. Records from six cores over 1 to 4 km reveal that the carbonate chemistry of the glacial North Atlantic was more stratified than the modern ocean, with higher [CO 3 2− ] by~20-30 μmol kg − 1 at 1-2 km and lower [CO 3 2− ] by~20 μmol kg − 1 at sites deeper than 3.5 km, producing an 800 m glacial shoaling of calcite saturation horizon. Comparison with benthic foraminiferal δ 13 C and ɛNd of Fe-Mn oxide leachates shows that the deep glacial waters with low-[CO 3 2− ] are consistent with a Southern Ocean source, while those with high-[CO 3 2− ] but distinct δ 13 C chemistry were supplied by two endemic sources, one being the Norwegian-Greenland Sea (NGS). Our carbonate ion results suggest that the glacial boundary between north and south sourced deep waters is at~2.8 km, significantly deeper than~2.3 km estimated from benthic foraminiferal δ 13 C. Weakened surface compensation inflow to the NGS promoted cooling and continental ice growth at north high-latitude, and a deeper boundary may reduce atmospheric CO 2 sequestration in the deep Atlantic, implying a greater role of other parts of the ocean.