Changes in North Atlantic deep-sea temperature during climatic fluctuations of the last 25,000 years based on ostracode Mg/Ca ratios (original) (raw)
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North Atlantic Deepwater Temperature Change During Late Pliocene and Late Quaternary Climatic Cycles
Science, 1995
Variations in the ratio of magnesium to calcium (Mg/Ca) in fossil ostracodes from Deep Sea Drilling Project Site 607 in the deep North Atlantic show that the change in bottom water temperature during late Pliocene 41,000-year obliquity cycles averaged 1.5^circC between 3.2 and 2.8 million years ago (Ma) and increased to 2.3^circC between 2.8 and 2.3 Ma, coincidentally with the intensification of Northern Hemisphere giaciation. During the last two 100,000-year glacial-to-interglacial climatic cycles of the Quaternary, bottom water temperatures changed by 4.5^circC. These results show that glacial deepwater cooling has intensified since 3.2 Ma, most likely as the result of progressively diminished deepwater production in the North Atlantic and of the greater influence of Antarctic bottom water in the North Atlantic during glacial periods. The ostracode Mg/Ca data also allow the direct determination of the temperature component of the benthic foraminiferal oxygen isotope record from Site 607, as well as derivation of a hypothetical sea-level curve for the late Pliocene and late Quaternary. The effects of dissolution on the Mg/Ca ratios of ostracode shells appear to have been minimal.
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.
Mid-Pliocene deep-sea bottom-water temperatures based on ostracode Mg/Ca ratios
Marine Micropaleontology, 2005
We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/ mol), and high latitude North Atlantic sites (552A, 610 and 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/ mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347Science 270, -1351 suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation. Published by Elsevier B.V.
Earth and Planetary Science Letters, 2015
Over the last decades extensive research has been carried out on changes in the Atlantic Meridional Overturning Circulation and its role during the last deglacial period. At present, only a few highresolution data sets from the deep eastern North Atlantic exist for this period. It therefore remains uncertain whether deepwater changes in the Eastern North Atlantic Basin were governed by alternating contributions of northern and southern deepwater or whether the Eastern North Atlantic Basin reflects changes in the initial composition and source of North Atlantic deepwater. Furthermore, it is still debated whether such changes are triggered by Northern or Southern Hemisphere climatic changes. In this centennial to decadal scale resolution study we investigate deepwater composition changes in the Eastern North Atlantic Basin over the last 15 ka BP. We used sediment cores GEOFAR KF16 and MD08-3180 (37.984 • N; 31.118 • W, wd 3050 m/37.999 • N; 31.134 • W, wd 3064 m), obtained from a small basin at the eastern flank of the Mid Atlantic Ridge south of the Azores Islands. Under modern conditions the coring site is situated at the boundary between southern Lower Deep Water and northern Eastern North Atlantic Deep Water consisting of Iceland Scotland Overflow Water and Labrador Sea Water. Distinct differences between the three water masses in terms of ventilation state, temperature and salinity signatures are ideal for tracking changes in the deglacial North Atlantic deepwater distribution using paired benthic foraminiferal stable isotopes (δ 13 C, δ 18 O) and Mg/Ca bottom water temperature reconstructions. The results show a close coupling of low bottom water temperature (1.5 • C) and δ 13 C (0-0.5h) values during cold Heinrich event 1, the Younger Dryas and the Preboreal, that were coeval with a major δ 18 O depletion of deepwater. The strong similarities between subtropical Eastern North Atlantic Deep Water and subpolar North Atlantic surface and deepwater changes indicate that deglacial changes in Eastern North Atlantic Deep Water distribution were triggered in the North Atlantic. Consequently, this would imply that changes in the North Atlantic also contributed to deglacial changes in Antarctic bottom water composition. During the early Holocene stepwise increasing δ 13 C values suggest increasing Eastern North Atlantic Deep Water production and/or increasing Eastern North Atlantic Deep Water ventilation with minor but distinct interruptions (at 10.8, 10.6, 9.1, 8.4, 8.1 and 7.2 ka BP), which were most probably also triggered in the subpolar North Atlantic.
Millennial-scale surface and subsurface paleothermometry from the northeast Atlantic, 55–8 ka BP
2008
We present high-resolution records of upper ocean temperatures derived from Mg/Ca ratios of surfacedwelling Globigerina bulloides and subsurface-dwelling Neogloboquadrina pachyderma sinistral and the relative abundance of N. pachyderma sinistral for the period 55-8 ka BP from NE Atlantic sediment core MD01-2461. Millennial-scale temporal variability and longer-term trends in these records enable us to develop a detailed picture of past ocean conditions such as a weakening of thermocline intensity from marine isotope stage 3 (MIS 3) to the last glacial maximum (LGM). The correspondence of all temperature proxies and convergence of paired oxygen isotope (d 18 O) records from both planktonic species implies a breakdown in the thermocline and year-round mixing of the upper water column through the LGM, perhaps related to decreasing insolation and additional cooling in association with the expansion of the circum-North Atlantic ice sheets. Millennialscale divergence in surface and subsurface temperatures and d 18 O across the last glacial correspond to meltwater release and the development of a strong halocline associated with both Heinrich (H) events and instabilities of the NW European ice sheet. During such episodes, G. bulloides Mg/Ca appears to record ambient, even warming summer sea surface temperatures across H events while the other proxies record maximum cooling.
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, 1994
Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3‐2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end‐member shifted from the Norwegian‐Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short‐term ventilation minimum below ...
Palaeogeography, Palaeoclimatology, Palaeoecology, 2000
Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe from Chain core 82-24-4PC from the western mid-Atlantic Ridge (3427 m) in order to estimate ocean circulation and bottom water temperature (BWT ) variability over the past 200,000 years. Mg/Ca ratios have been used as a paleothermometer because the ratios are controlled primarily by ambient water temperatures at the time the organism secretes its adult carapace. Over the past two glacial-interglacial cycles, Mg/Ca values oscillated between about 7 mmol/mol and 12 mmol/mol, equivalent to a BWT range of 0 to >3.5°C. The lowest values were obtained on specimens from glacial marine isotope stages (MISs) 2, 4 and 6; the highest values were obtained from specimens from the early part of the Holocene interglacial (MIS 1), and also from MISs 5 and 7. These trends suggest that BWTs in the North Atlantic Ocean fluctuate over orbital time scales.
Quaternary Science Reviews, 2019
Variations in North Atlantic Ocean mid-depth circulation during the Holocene are poorly understood. It is believed that they had a significant influence on the properties of water entering the Nordic Sea by redistributing heat and freshwater, potentially affecting deep-water formation and climate. To improve our knowledge of the NE Atlantic mid-depth circulation, radiocarbon and neodymium isotope analyses have been then carried out on precisely dated (U-Th) L. pertusa and M. oculata coral fragments from two sediment cores taken at ~750 m water depth on the SW Rockall Trough margin. Cold-water coral (CWC) εNd values vary between-12.2±0.3 and-16.6±0.4 and result from variable contributions of unradiogenic mid-depth subpolar gyre (mid-SPG) water (~-15) and more radiogenic Eastern North Atlantic Water (ENAW) (~-11) which is transported northward to the Rockall Trough by boundary currents along the European margin. Increased coral εNd reflects a westward contraction of the mid-SPG water and a higher proportion of ENAW. The mid-Holocene (from 8.8 to 6.8 ka BP) is marked by unradiogenic coral εNd (from-16.6±0.4 to-14.6±0.5) indicating a greater eastward extension of the mid-SPG. This is followed by a shift from 6.8 to 5 ka BP toward more radiogenic εNd