ENSO and interdecadal climate variability over the last century documented by geochemical records of two coral cores from the South West Pacific

The south west Pacific is affected by climatic phenomena such as ENSO (El Ni&#241;o Southern Oscillation) or the PDO (Pacific Decadal Oscillation). Near-monthly resolution calibrations of Sr/Ca, U/Ca and <span style=&quot;font-family: &apos;Symbol, Times New Roman, Times&apos;; font-size: 16px;&quot;>&delta;</span>¹⁸Oc were made on corals taken from New Caledonia and Wallis Island. These geochemical variations could be linked to SST (sea surface temperature) and SSS (sea surface salinity) variations over the last two decades, itselves dependent on ENSO occurrences. On the other hand, near-half-yearly resolution over the last century smoothes seasonal and interannual climate signals, but emphasizes low frequency climate variability

An ocean–ice coupled response during the last glacial: a view from a marine isotopic stage 3 record south of the Faeroe Shetland Gateway

150 μm) lithic fraction (grain concentration) and the analysis of selected biogenic proxies (assemblages and stable isotope ratio of calcareous planktonic foraminifera, dinoflagellate cyst – e.g. dinocyst – assemblages). Results presented here are focussed on the dinocyst response, this proxy providing the reconstruction of past sea-surface hydrological conditions, qualitatively as well as quantitatively (e.g. transfer function sensu lato). Our study documents a very coherent and sensitive oceanic response to the MIS3 rapid climatic variability: strong fluctuations, matching those of stadial/interstadial climatic oscillations as depicted by Greenland ice cores, are recorded in the MD99-2281 archive. Proxies of terrigeneous and detritical material suggest increases in continental advection during Greenland Stadials (including Heinrich events), the latter corresponding also to southward migrations of polar waters. At the opposite, milder sea-surface conditions seem to develop during Greenland Interstadials. After 30 ka, reconstructed paleohydrological conditions evidence strong shifts in SST: this increasing variability seems consistent with the hypothesised coalescence of the British and Fennoscandian ice sheets at that time, which could have directly influenced sea-surface environments in the vicinity of core MD99-2281

Hurricanes and climate in the Caribbean during the past 3700 years BP

International audienceA multiproxy analysis of lacustrine sediments cored in Grand-Case Pond at Saint-Martin, north of the Lesser Antilles archipelago, reveals three distinct climatic periods for the last 3700 years. From 3700 to ~2500 yr cal. BP and from 1150 yr cal. BP to the present, carbonate mud deposition occurred in connection with pond lowstands. These periods were also punctuated by severe drought events, marked by gypsum laminae, and hurricane landfalls, leading to marine sand inputs into the pond. The intermediate time interval, from 2500 to 1150 yr cal. BP, is typified by black organic mud deposition, suggesting that hypoxic to anoxic conditions prevailed at the pond bottom. These were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today. The carbon isotopic composition of the ostracod Perissocytheridea bisulcata shows that the lowest δ13C values are recorded during the hypoxic periods, as a consequence of bacterial recycling of isotopically depleted organic matter. Such a climatic history agrees closely with that documented from other records in the Caribbean area, such as the Cariaco Basin, central coast of Belize or Barbados. By constrast, discrepancies seem to emerge from the comparison between hurricane activity recorded at Saint-Martin on the one hand and Vieques (Puerto Rico) on the other hand. We explain this apparent contradiction by a balance between two distinct storm paths in response to latitudinal shifts of the Intertropical Convergence Zone (ITCZ). Stronger storm activity over the Gulf coast and the inner Caribbean Sea is favoured by a southern position of the ITCZ in connection with dry climatic conditions. Plausible links with the North Atlantic Oscillation (NAO) are also suggested

Stratification of surface waters during the last glacial millennial climatic events: a key factor in subsurface and deep-water mass dynamics

The last glacial period was punctuated by abrupt climatic events with extrema known as Heinrich and Dansgaard–Oeschger events. These millennial events have been the subject of many paleoreconstructions and model experiments in the past decades, but yet the hydrological processes involved remain elusive. In the present work, high-resolution analyses were conducted on the 12–42 ka BP section of core MD99-2281 retrieved southwest of the Faeroe Islands, and combined with analyses conducted in two previous studies (Zumaque et al., 2012; Caulle et al., 2013). Such a multiproxy approach, coupling micropaleontological, geochemical and sedimentological analyses, allows us to track surface, subsurface, and deep hydrological processes occurring during these rapid climatic changes. Records indicate that the coldest episodes of the studied period (Greenland stadials and Heinrich stadials) were characterized by a strong stratification of surface waters. This surface stratification seems to have played a key role in the dynamics of subsurface and deep-water masses. Indeed, periods of high surface stratification are marked by a coupling of subsurface and deep circulations which sharply weaken at the beginning of stadials, while surface conditions progressively deteriorate throughout these cold episodes; conversely, periods of decreasing surface stratification (Greenland interstadials) are characterized by a coupling of surface and deep hydrological processes, with progressively milder surface conditions and gradual intensification of the deep circulation, while the vigor of the subsurface northward Atlantic flow remains constantly high. Our results also reveal different and atypical hydrological signatures during Heinrich stadials (HSs): while HS1 and HS4 exhibit a "usual" scheme with reduced overturning circulation, a relatively active North Atlantic circulation seems to have prevailed during HS2, and HS3 seems to have experienced a re-intensification of this circulation during the middle of the event. Our findings thus bring valuable information to better understand hydrological processes occurring in a key area during the abrupt climatic shifts of the last glacial period

Sea surface salinity reconstruction as seen with foraminifera shells: Methods and cases studies

Reconstruction of past salinities in surface oceans (SSS) can be done by measuring the isotopic composition of foraminifera shells found in the deep sea sediments. The proportion of heavy oxygen isotopes (18O) in the calcite of these shells depend on the temperature and the isotopic oxygen composition of the surrounded waters (δ18 Osw), this latter parameter depending on the water salinity. Mainly two equations allows to reconstructed past SSS, one estimating past temperature variations and the other one changes in the δ18 Osw through time. Uncertainties linked with these calculation can be important, and therefore quantitative reconstructions need to be taken with cautions. For some specific cases, uncertainties on temperature and δ18 Osw estimations can be reduced. For such cases, salinity reconstructions showing amplitude changes higher than 1 per mil can be considered as significative

Relation between low latitude insolation and δ 18 O change of atmospheric oxygen for the last 200 kyrs, as revealed by Mediterranean sapropels

International audienceThe isotopic ratio of atmospheric O2, δ18Oatm, deduced from ice cores, displays large fluctuations during climatic cycles. These are caused principally by changes in the oxygen isotopic ratio of sea-water, δ18Osw, and changes in the biosphere and in the hydrological cycle. As both δ18Oatm and δ18Osw coincide closely over the last 135 kyr BP, it is generally believed that δ18Oatm is driven mainly by δ18Osw. Here we focus on the major discrepancy, which arises between those two signals during the prior isotopic glacial stage 6, around 175 kyr BP, discrepancy which calls into question the role of changes in δ18Osw as the driving mechanism for δ18Oatm. We present arguments, based on the occurrence and pollen content of Mediterranean sapropels, for another source of the δ18Oatm change: insolation at low latitudes via hydrosphere/biosphere activity

The Dole effect over the last two glacial-interglacial cycles

International audienceDetailed measurements of filSO of atmospheric oxygen performed on air trapped in the Vostok ice cores (Antarctica) are used to extend the record of the Dole effect over two climatic cycles (back to 240 kyr B.P.). Except for glacial terminations I and II and for an unexpected minimum occurring around 175 kyr, the Dole effect shows small variations (ADole within + 0.5%o). These small variations, however, show a well-marked 23 kyr precessional periodicity, thus confirrning the results obtained by Bender et al. [ 1994a] for the first climatic cycle. To explain the minimum value reached around 175 kyr, we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions