Temperature and salinity reconstruction for the Last Interglacial Period in the North Atlantic Ocean
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Marine localities on the west European shelf have been studied to reconstruct the nearshore palaeoceanography of the last two millennia. The sites form a transect from the Iberian margin northeastward via Scotland to western Norway and Iceland. Proxies used for palaeoclimatic reconstructions include stable isotopes, benthic and planktonic foraminfera, diatoms, dinoflagellates, as well as geochemical and sedimentological parameters. Major changes as well as long-term trends in oceanographic conditions are observed in the records, including a general cooling trend through much of the last millennium. There is a clear linkage between the atmospheric processes and the oceanic circulation, and the ocean temperature variability in the records can be correlated with the so-called ‘Mediaeval Warm Period’ and ‘Little Ice Age’. These oscillations are, however, by no means unique within the last two millennia. As an example, sea surface temperatures to the north of Iceland and on the Iberian margin were higher in the Roman Warm Period than at any time during the ‘Mediaeval Warm Period’. However, the palaeoceanographic record generally supports a distinct cooling at the transition between the ‘Mediaeval Warm Period’ and the ‘Little Ice Age’. While a number of records indicate a warming of coastal and shelf waters during the last 200 years, the twentieth century does not appear to be unusual when the proxy records spanning the last two millennia are examined.
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<p>Interglacial climate conditions are generally characterized by relatively strong and persistent deep-water formation both in the North Atlantic and in the Southern Ocean, and overall &#8216;stable&#8217; climate conditions. Recent evidence, however, challenges the notion of persistent deep-water formation in both hemispheres during the last interglacial, and points at rapid reductions in convective mixing that may have lasted few centuries to millennia. The spatial pattern of this phenomenon and its driving mechanisms remain poorly constrained. Here we present multi-proxy data for rapid reductions in bottom water oxygen in the central sub-Antarctic Atlantic (sediment core MD07-3077, 44&#176;9.20&#8217;S, 14&#176;13.69&#8217;W, 3776 m water depth) during the warmer-than-present period of the last interglacial (i.e., 132-116 kyr before present). The first of these &#8220;stagnation events&#8221;, as they are often denoted, is synchronous, within dating uncertainties, with a similar drop in bottom water oxygenation at a more southern site, ODP Site 1094, south of the Polar Front. Our findings hint at a widespread and significant change in the formation rate and/or end-member pre-formed oxygen levels of Antarctic bottom water (AABW) in the South Atlantic during the last interglacial. The onset of these events closely coincides with increases in sea surface temperatures in the sub-Antarctic Atlantic above average Holocene levels. Although this needs to be further tested at more proximal sites, we argue that stagnation events were likely driven by excess ocean warming, in particular below ice shelves in the Weddell Sea, that may have perturbed AABW formation and/or air-sea gas exchange in that region during the last interglacial. Our findings highlight important feedback mechanisms linking hydrographic conditions at the sea surface, instabilities of the local cryosphere, and the strength of deep water formation in warmer-than-present climate scenarios &#8211; the full understanding of which has relevance for assessing the trajectory of future changes in the Southern Ocean.</p>
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Abstract Interglacial periods (IG) offer an opportunity to understand natural climate variability and its drivers under potential warmer‐than‐present conditions. However, sea‐surface temperature (SST) records from the Southern Ocean (SO) are limited. The first SST record from the Sub‐Antarctic western Indian SO covering the last four IGs suggest warmer conditions during Marine Isotope Stage 5e than 9e, 7e, and Holocene. Each IG presents two (early and late) warm phases interrupted by a cooling, except Holocene that experienced a continuous warming. The early warm phase might be attributable to changes in northern summer insolation with feedbacks from Northern and Southern Hemisphere ice‐sheets, global oceanic circulation, and the carbon cycle. Conversely, the late warm phase might be due to changes in Southern Hemisphere summer insolation. Larger millennial‐scale SST variability for IGs older than Holocene could be attributed to a less stable thermohaline circulation, which resulted in more variable heat redistribution between the two hemispheres.
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Past sea surface temperatures (SST) in the northern and southern areas of the South China Sea have been reconstructed for the past 220 kyr using the U K 37 alkenone index. The SST profiles follow the glacial/interglacial pattern exhibiting differences between Last Glacial Maximum and Holocene that are 1°–3°C larger than those observed at the same latitudes in the Atlantic and Pacific Oceans. In Termination I both planktonic foraminiferal δ 18 O and SST exhibit well‐defined Bølling‐Allerød and Younger Dryas events with temperature differences between both periods of 0.8° and 0.4°C in north and south, respectively. SSTs record a constant north‐south difference of 1°C in the interglacials and nearly 2.5°C in the glacial stages. These differences define two distinct climatic and water circulation patterns that correspond with glacial/interglacial sea level oscillations which opened and closed water exchange with the tropical Indo‐Pacific Ocean through the present Sunda Shelf.
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Similar orbital geometry and greenhouse gas concentrations during marine isotope stage 11 (MIS 11) and the Holocene make stage 11 perhaps the best geological analogue period for the natural development of the present interglacial climate. Results of a detailed study of core MD01‐2443 from the Iberian margin suggest that sea surface conditions during stage 11 were not significantly different from those observed during the elapsed portion of the Holocene. Peak interglacial conditions during stage 11 lasted nearly 18 kyr, indicating a Holocene unperturbed by human activity might last an additional 6–7 kyr. A comparison of sea surface temperatures (SST) derived from planktonic foraminifera for all interglacial intervals of the last million years reveals that warm temperatures during peak interglacials MIS 1, 5e, and 11 were higher on the Iberian margin than during substage 7e and most of 9e. The SST results are supported by heavier δ 18 O values, particularly during 7e, indicating colder SSTs and a larger residual ice volume. Benthic δ 13 C results provide evidence of a strong influence of North Atlantic Deep Water at greater depths than present during MIS 11. The progressive ocean climate deterioration into the following glaciation is associated with an increase in local upwelling intensity, interspersed by periodic cold episodes due to ice‐rafting events occurring in the North Atlantic.
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