Decadal changes in the mid-depth water mass dynamic of the Northeastern Atlantic margin (Bay of Biscay) (original) (raw)
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The climate influence on the mid-depth Northeast Atlantic gyres viewed by cold-water corals
The neodymium (Nd) isotopic composition (expressed in epsilon units, eNd) of reef framework-forming cold-water corals provides unique measures of water mass provenance and mixing within the Northeast Atlantic today and in the past. A reconstruction of near thermocline water eNd from cold-water corals of the Gulf of Cádiz and Porcupine Seabight spanning over the past 300,000 years, now revealed that climate cooling during Marine Isotope Stages (MIS) 7.2 and MIS 8/9 led to a retraction of the mid-depth Subpolar Gyre (mSPG) to the west. Conversely, Northern Hemisphere warming and increasing fresh water fluxes to the northwest (Labrador Sea) favor a stronger eastward extension of the mSPG blocking the northward flow of temperate Atlantic water as observed during the early MIS 1 and the early stage MIS 5.5. These changes are likely the result of large-scale south-north displacement of the westerlies similar to present-day observations that the North Atlantic Oscillation (NAO) is linked with mid-depth ocean circulation. Based on these observations, we hypothesize that further climate warming will also strengthen the mSPG leading to a salt and temperature decrease in the Northeast Atlantic whereas salinity and temperature will increase in the temperate Atlantic. However, the amplitude of such changes on North Atlantic overturning remains to be tested.
Geochemistry, Geophysics, Geosystems, 2000
1] Abstract: We reconstructed three time series of last glacial-to-present deep-sea temperature from deep and intermediate water sediment cores from the western North Atlantic using Mg/Ca ratios of benthic ostracode shells. Although the Mg/Ca data show considerable variability (``scatter'') that is common to single-shell chemical analyses, comparisons between cores, between core top shells and modern bottom water temperatures (BWT), and comparison to other paleo-BWT proxies, among other factors, suggest that multiple-shell average Mg/Ca ratios provide reliable estimates of BWT history at these sites. The BWT records show not only glacial-to-interglacial variations but also indicate BWT changes during the deglacial and within the Holocene interglacial stage. At the deeper sites (4500-and 3400-m water depth), BWT decreased during the last glacial maximum (LGM), the late Holocene, and possibly during the Younger Dryas. Maximum deep-sea warming occurred during the latest deglacial and early Holocene, when BWT exceeded modern values by as much as 2.58C. This warming was apparently most intense around 3000 m, the depth of the modern-day core of North Atlantic deep water (NADW). The BWT variations at the deeper water sites are consistent with changes in thermohaline circulation: warmer BWT signifies enhanced NADW influence relative to Antarctic bottom water (AABW). Thus maximum NADW production and associated heat flux likely occurred during the early Holocene and decreased abruptly around 6500 years B.P., a finding that is largely consistent with paleonutrient studies in the deep North Atlantic. BWT changes in intermediate waters (1000-m water depth) of the subtropical gyre roughly parallel the deep BWT variations including dramatic mid-Holocene cooling of around 48C. Joint consideration of the Mg/Ca-based BWT estimates and benthic oxygen isotopes suggests that the cooling was accompanied by a decrease in salinity at this site. Subsequently, intermediate waters warmed to modern values that match those of the early Holocene maximum of $78C. Intermediate water BWT changes must also be driven by changes in ocean circulation. These results thus provide independent evidence that supports the hypothesis that deepocean circulation is closely linked to climate change over a range of timescales regardless of the mean climate state. More generally, the results further demonstrate the potential of benthic Mg/Ca ratios as a tool for reconstructing past ocean and climate conditions.
Pages …, 2003
Climate modeling studies predict that anthropogenic increases in greenhousegas concentrations will possibly cause a weakening or even a shut-down of the meridional overturning circulation in the Atlantic (thermohaline circulation, THC), through global warming and an intensification of the hydrological cycle . Therefore it is essential to monitor the Atlantic THC, preferably on a permanent basis. Present field observations of the THC, however, are insufficient to detect whether its strength is changing. Climate models exhibit pronounced and rapid warming of the tropical intermediate-depth Atlantic Ocean in consequence of a THC slowdown, suggesting that mid-depth Atlantic temperatures may serve as an indicator of THC change. Applying different forcings to an ocean general circulation model, representing present-day and glacial climates, we show that this mid-depth water response is a robust feature in both climatic situations . Given that dramatic changes of the THC occurred during the last deglaciation, the reconstruction of Atlantic intermediate-depth temperatures from sediment cores provides an opportunity to evaluate the reliability of the model simulations and the suitability of tropical mid-depth Atlantic temperature change as a tracer of THC strength. For this purpose we studied two sediment cores recovered from high accumulation areas, southeast of the island of
Geochemistry, Geophysics, Geosystems, 2016
The Nd isotopic composition (εNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cádiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain mid-depth water mass dynamics within the Gulf of Cádiz over the past 40 ka. εNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cádiz reveals relatively constant values (-8.6 to -9.0 and -9.5 to -10.4, respectively). Such values are similar to those of the surrounding present-day mid-depth waters from the Mediterranean Outflow Water (MOW; εNd ~ -9.4) and Mediterranean Sea Water (MSW; εNd ~ -9.9). In contrast, glacial εNd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of Cádiz display a higher average value (-8.9±0.4) compared to the present-day value (-11.7±0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic εNd values (~ -8) at 19, 23-24 and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW εNd values remained stable during the last glacial period, it is suggested that these radiogenic εNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.
Earth and Planetary Science Letters
Radiogenic neodymium isotopes have been used as a water mass mixing proxy to investigate past changes in ocean circulation. Here we present a new depth transect of deglacial neodymium isotope records measured on uncleaned planktic foraminifera from five cores spanning from 3300 to 4900 m on the Mauritanian margin, in the tropical eastern Atlantic as well as an additional record from 4000 m on the Ceara Rise in the equatorial western Atlantic. Despite being located under the Saharan dust plume, the eastern Atlantic records differ from the composition of detrital inputs through time and exhibit similar values to the western Atlantic foraminiferal Nd across the deglaciation. Therefore we interpret the foraminiferal values as recording deep water Nd isotope changes. All six cores shift to less radiogenic values across the deglaciation, indicating that they were bathed by a lower proportion of North Atlantic Deep Water during the Last Glacial Maximum (LGM) relative to the Holocene. The eastern Atlantic records also show that a neodymium isotope gradient was present during the LGM and during the deglaciation, with more radiogenic values observed at the deepest sites. A homogeneous water mass observed below 3750 m in the deepest eastern Atlantic during the LGM is attributed to the mixing of deep water by rough topography as it passes from the western Atlantic through the fracture zones in the Mid-Atlantic Ridge. This implies that during the LGM the low latitude deep eastern Atlantic was ventilated from the western Atlantic via advection through fracture zones in the same manner as occurs in the modern ocean. Comparison with carbon isotopes indicates there was more respired carbon in the deep eastern than deep western Atlantic during the LGM, as is also seen in the modern Atlantic Ocean.
Southwest Atlantic watermass evolution during the last deglaciation
The rise in atmospheric CO 2 during Heinrich Stadial 1 (HS1; 14.5-17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind-driven upwelling in the Southern Ocean can liberate 13 C-depleted carbon from the abyss, causing atmospheric CO 2 to increase and the δ 13 C of CO 2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13 C-depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ 18 O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ 18 O signal suggests that correcting δ 18 O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid-depth δ 13 C minima were most likely driven by an abrupt drop in δ 13 C of northern component water. Low δ 13 C at the Brazil Margin also coincided with an~80‰ decrease in Δ 14 C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO 2 during HS1. 13 C-enriched values at some point during the deglaciation , the precise timing of the shift has remained poorly constrained. Here we evaluate the timing using a depth transect of 12 cores from the Brazil Margin spanning 400 m to 4000 m water depth ). Each core is placed into a common chronologic framework to minimize relative differences in timing, and the resulting time series are cross checked using records at adjacent water depths. Vertical profiles of δ 13 C and δ 18 O at discrete time steps are then used to evaluate the timing of circulation changes in different parts of the water column.
Reorganization of the upper ocean circulation in the mid-Holocene in the northeastern Atlantic 1,2
A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge (RR; core LO09-14; 59812.30'N, 31805.94'W) and the Faroe–Shetland Channel (FSC; core HM03-133-25; 60806.55'N, 06804.18'W) to document hydrographical changes of the North Atlantic Current (NAC) during the Holocene. Dinocyst and coccolith assemblages were analyzed, and quantitative reconstructions of sea surface temperatures (SSTs) and sea surface salinities (SSSs) were conducted based on dinocyst assemblages. Both proxies suggest a major reorganization of surface circulation patterns in the northeastern North Atlantic between 7 and 5.4 ka BP. At both sites, SSSs before 6.5–7 ka BP were lower than during the mid-late Holocene, suggesting dispersal of meltwater through the NAC. Long term trends of SSTs, however, show higher than present summer SSTs on the RR from 9.3 to *6 ka BP, and lower than present SSTs in the FSC until ca. 5.4 ka BP. The contrasted SST trends at the two sites suggest that decreasing summer insolation was not the only forcing behind hydrographical changes in the region. Decoupling of the NAC and the Slope Current (SC), which both influence the FSC, is proposed as a possible mechanism. We hypothesize that a strong NAC during the early to middle Holocene resulted in a SST increase on the RR and decrease in the FSC. Inversely, a weaker NAC after 5–6 ka BP, leading to decreased SSTs on the RR, would have enhanced the relative contribution of the warmer, saltier SC in the FSC, thus resulting in a regional SST and SSS increase.
Paleoceanography, 1995
Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (>8 cm/kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The •5180 and •513C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 •4C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during •80 event 3.1 and the B611ing. Meltwater phases markedly influenced the European continental climate by suppressing the "heat pump" of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell. Introduction The circulation regime in the Nordic Seas has a strong impact on the climate of the adjacent continents. Today, the warm waters of the North Atlantic Drift, the Irminger Current, Paper number 95PA01453. 0883-8305/95/95PA-01453 $10.00
Marine Geology, 2019
Variations of the Sub-Polar Gyre (SPG) and the SubTropical Gyre (STG) circulation during the Holocene are believed to be related to regional and global climate over this time period. To improve our understanding of these phenomena we provide new constraints on variations in surface circulation patterns using neodymium isotopes (εNd) on precisely U-Th dated coral fragments of L. pertusa. The fragments were retrieved from two sediment cores taken from cold-water coral (CWC) mounds at ~ 127-134 m water depth in the Mingulay Reef Complex located on the Western British continental shelf. The results have been combined with εNd analyzed on seawater samples from two stations located on the continental shelf and margin in order to establish whether εNd is a reliable proxy of the ocean circulation variations and