Holocene evolution of summer winds and marine productivity in the tropical Indian Ocean in response to insolation forcing: data-model comparison

The relative abundance of <i>Globigerinoides bulloides</i> was used to infer Holocene paleo-productivity changes on the Oman margin and at the southern tip of India. Today, the primary productivity at both sites reaches its maximum during the summer season, when monsoon winds result in local Eckman pumping, which brings more nutrients to the surface. On a millennium time-scale, however, the % <i>G. bulloides</i> records indicate an opposite evolution of paleo-productivity at these sites through the Holocene. The Oman Margin productivity was maximal at ~9 ka (boreal summer insolation maximum) and has decreased since then, suggesting a direct response to insolation forcing. On the contrary, the productivity at the southern tip of India was minimum at ~9 ka, and strengthened towards the present. <br><br> Paleo-reconstructions of wind patterns, marine productivity and foraminifera assemblages were obtained using the IPSL-CM4 climate model coupled to the PISCES marine biogeochemical model and the FORAMCLIM ecophysiological model. These reconstructions are fully coherent with the marine core data. They confirm that the evolution of particulate export production and foraminifera assemblages at our two sites were directly linked with the strength of the upwelling. Model simulations at 9 ka and 6 ka BP show that the relative evolution between the two sites since the early Holocene can be explained by the weakening but also the southward shift of monsoon winds over the Arabian Sea during boreal summer

High-resolution sea surface reconstructions off Cape Hatteras over the last 10 ka

International audienceThis study presents high-resolution foraminiferal-based sea surface temperature, sea surface salinity and upper water column stratification reconstructions off Cape Hatteras, a region sensitive to atmospheric and thermohaline circulation changes associated with the Gulf Stream. We focus on the last 10,000 years (10 ka) to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability. We observed opposite evolutions between the conditions off Cape Hatteras and those south of Iceland, known today for the North Atlantic Oscillation pattern. We interpret the temperature and salinity changes in both regions as co-variation of activities of the subtropical and subpolar gyres. Around 8.3 ka and 5.2-3.5 ka, positive salinity anomalies are reconstructed off Cape Hatteras. We demonstrate, for the 5.2-3.5 ka period, that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north. A northward displacement of the Gulf Stream, blocking the southbound low-salinity flow, concomitant to a reduced Meridional Overturning Circulation is the most likely scenario. Finally, wavelet transform analysis revealed a 1000-year period pacing the δ18O signal over the early Holocene. This 1000-year frequency band is significantly coherent with the 1000-year frequency band of Total Solar Irradiance (TSI) between 9.5 ka and 7 ka and both signals are in phase over the rest of the studied period

Révision du paléothermomètre Mg/Ca et son application sur l'hydrologie de surface de l'océan indien tropical au cours de l'Holocène

Nous avons montré l influence de la salinité sur le paléothermomètre Mg/Ca dans des zones tropicales. Une procédure de correction est développée afin de dériver des températures de surface (SST) et des delta 18Osw de l eau non biaisés par l effet de la salinité à partir des mesures de température dérivées du Mg/Ca (TMg/Ca) et du delta18O de la calcite de foraminifère. Cette procédure de correction a été appliquée pour étudier la variabilité de la mousson indienne au cours de l Holocène (derniers ~ 10 ka). Les reconstructions hydrologiques sont basées sur les mesures couplées de delta18O et de Mg/Ca sur un foraminifère planctonique de surface Globigerinoides ruber (G. ruber). Nous avons reconstruit l évolution des gradients hydrographiques Est/Ouest et Nord/Sud de l Océan Indien. L étude de la variabilité de la circulation atmosphérique saisonnière de l Océan Indien à partir d enregistrements sédimentaires situés sous trois upwellings induits par la mousson d été et d hiver, montre que ces upwellings réagissent à des forçages différents. Nous interprétons la dynamique de ces upwellings par la réduction progressive de l insolation, ou par le déplacement de la Zone de convergence Intertropicale (ITCZ), ou encore par la température atmosphérique de l hémisphère sud. L étude de la variabilité rapide des conditions hydrographiques de surface du sud-est de la Mer Rouge montre plusieurs événements de forte salinité qui semblent se produire avec une périodicité de ~ 1000 ans.We show the salinity effect on sea surface Mg/Ca temperature (TMg/Ca) reconstructions from core-tops located in tropical oceans. We develop a correction procedure to derive un-biaised SST and delta18Osw from TMg/Ca and delta18O of foraminiferal calcite. We apply this correction procedure to study the Indian monsoon variability during the Holocene solar insolation changes (last ~ 10 ka). The surface hydrographic reconstructions are based on coupled delta18O-Mg/Ca measurements performed on the planktic foraminifera Globigerinoides ruber (G. ruber). We reconstruct the evolution of hydrographic gradients (East/West and North/South) over the Holocene. We also study the seasonal atmospheric circulation variability by analysing changes in the intensity of three local monsoon-driven upwellings recorded in marine sediment cores from the northern tropical Indian Ocean. These upwellings respond to summer and winter monsoon forcings. Our results suggest that these upwellings react to different forcings. We interpret the upwelling dynamics as a response to the progressive reduction of northern hemisphere solar insolation or a shift of the Intertropical Convergence Zone (ITCZ) summer position, or atmospheric temperature of the southern hemisphere. The study of millennial hydrographic variability in the south-east of the Red Sea shows more saline events evenly spaced which seem occur with a ~ 1000 yr periodicity.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Salinity bias on the foraminifera Mg/Ca thermometry: Correction procedure and implications for past ocean hydrographic reconstructions

International audience[1] Mg/Ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures. Here we show, however, that the Mg/Ca temperature relationship of the planktonic species Globigerinoides ruber is significantly affected by seawater salinity, with a +1 psu change in salinity resulting in a +1.6°C bias in Mg/Ca temperature calculations. If not accounted for, such a bias could lead, for instance, to systematic overestimations of Mg/Ca temperatures during glacial periods, when global ocean salinity had significantly increased compared to today. We present here a correction procedure to derive unbiased sea surface temperatures (SST) and d 18 O sw from G. ruber T Mg/Ca and d 18 O f measurements. This correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the Last Glacial Maximum (LGM) in the Western Pacific Warm Pool. While uncorrected T Mg/Ca data indicate a 3°C warming of the Western Pacific Warm Pool since the LGM, the salinity-corrected SST result in a stronger warming of 4°C. Components: 10,959 words, 5 figures, 5 tables

Field-based validation of a diagenetic effect on G. ruber Mg/Ca paleothermometry: Core top results from the Aegean Sea (eastern Mediterranean)

Recent work across the Mediterranean Sea has illustrated the salinity and overgrowth effects on planktonic foraminiferal Mg/Ca, which potentially confound the use of this as a temperature proxy for paleoceanographic reconstructions. To test and verify these effects, we present new Aegean Sea results which reveal Mg/Ca values that were unreasonably high to be explained by temperature or salinity variations alone, confirming that foraminiferal Mg/Ca is affected by diagenesis. We have specifically targeted Globigerinoides ruber (w, sensu stricto), from a series of modern core tops spanning a strong sea surface salinity gradient and a minor sea surface temperature range, along a north?south Aegean Sea transect. Scanning Electron Microscopy analyses show that G. ruber specimens were covered by microscale euhedral crystallites of inorganic precipitates. This secondary calcite phase seems to be responsible for the anomalously high Mg/Ca ratios and likely formed near the sediment/water interface from CaCO3 supersaturated interstitial seawater. We also have clear evidence of diagenetic alteration in a north?south direction along the Aegean Sea, possibly depending on salinity and calcite saturation state gradients. These observations illustrate the necessity of alternative techniques (e.g., flow?through time resolved analysis or laser ablation inductively coupled plasma mass spectrometry) to potentially overcome these diagenetic issues and develop a more reliable and sensitive temperature proxy in similar subtropical settings characterized by high salinity, excessive evaporation, and restricted circulation

Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing

International audienceDuring the Last Glacial Maximum, tropical sea surface temperatures were 1 to 3 degrees C cooler than present(1-4), but the altitude of the snowlines of tropical glaciers(5,6) was lower than would be expected in light of these sea surface temperatures. Indeed, both glacial and twentieth-century snowlines seem to require lapse rates that are steeper than a moist adiabat(7,8). Here we use estimates of Last Glacial Maximum sea surface temperature in the Indo-Pacific warm pool based on the clumped isotope palaeotemperature proxy in planktonic foraminifera and coccoliths, along with radiative-convective calculations of vertical atmospheric thermal structure, to assess the controls on tropical glacier snowlines. Using extensive new data sets for the region, we demonstrate that mean environmental lapse rates are steeper than moist adiabatic during the recent and glacial. We reconstruct glacial sea surface temperatures 4 to 5 degrees C cooler than modern. We include modern and glacial sea surface temperatures in calculations of atmospheric convection that account for mixing between rising air and ambient air, and derive tropical glacier snowlines with altitudes consistent with twentieth-century and Last Glacial Maximum reconstructions. Sea surface temperature changes <= 3 degrees C are excluded unless glacial relative humidity values were outside the range associated with deep convection in the modern. We conclude that the entrainment of ambient air into rising air masses significantly alters the vertical temperature structure of the troposphere in modern and ancient regions of deep convection. Furthermore, if all glacial tropical temperatures were cooler than previously estimated, it would imply a higher equilibrium climate sensitivity than included in present models(9,10)