Alkenone distributions in the North Atlantic and Nordic sea surface waters

International audienceThe C37–C39 alkenones were quantified in suspended particulate matter obtained from the surface waters of the North Atlantic, including the Nordic seas, over a temperature range of 4°–20°C. U37K′ values were linearly correlated to temperature over the entire range covered by our data set, i.e., 4°–20°C (U37K′ = 0.027T + 0.036, r2 = 0.97). The compilation of water column data from the Atlantic, Pacific, and Southern Ocean and Mediterranean Sea suggests that the linear model may not be the best function for fitting U37K′ and in situ temperature values. It also shows that suspended matter data from warm waters (T > 20°C) are still needed to constrain the equation of the global curve. High abundances of C37:4 were found in the coldest polar waters. Around 4°C, %C37:4 represented up to 35%, a value that dropped to zero at temperatures above 10°C. Values of %C37:4 were linearly correlated to temperature (%C37:4 = −3.7T + 43.7, r2 = 0.50) and salinity (%C37:4 = −48.1S‰ + 1691, r2 = 0.78). Further investigations from other oceanic basins are necessary to confirm these finding

Drastic changes in deep-sea sediment porewater composition induced by episodic input of organic matter

We report the first seasonal time-series observations of porewater composition obtained for deep-sea sediments. We observed considerable temporal variability of O-2, NO3-, and Mn2+ profiles at the French JGOFS site DYFAMED (western Mediterranean) in response to a pulsed input of organic matter. A delivery of reactive organic matter representing only 1% of the average organic C content of surface sediments (0.6% wt/wt) was required in order to fuel the observed changes in porewater composition. The perturbation resulted in dramatic changes in integrated reaction rates and sediment-water fluxes. O-2 uptake rates increased from 1.26 to 1.82 mmol m(-2) d(-1), while nitrification rates rose from 0.13 to 0.18 mmol m(-2) d(-1). These changes were paralleled by a decrease in NO3- effluxes across the sediment-water interface from 0.07 to 0.01 mmol m(-2) d(-1) and a concomitant rise of denitrification rates from 0.06 to 0.17 mmol m(-2) d(-1). Porewater profiles responded rapidly to the sediment pulse deposition, returning to their steady-state values within a few months. The main driving force appeared not to be the seasonal variability of pelagic production but rather the downslope transport of resuspended upper-shelf sediments

A 27 kyr terrestrial biomarker record in the southern Indian Ocean

Terrestrial inputs were reconstructed using high molecular weight n-alkane concentrations recorded in the sub-Antarctic Indian Ocean core MD94-103 (45 degrees 35'S, 86 degrees 31'E, 3560 m) to examine regional changes in the atmospheric circulation over the last 27 kyr. This record was compared to the dust content of EPICA-Dome C ice and continental data from South Africa ( e. g., pollen sequences and isotope records in speleothems) to get a comprehensive understanding of atmospheric links between low and midlatitudes of the Indian Ocean. Terrestrial n-alkanes indicate higher glacial than Holocene inputs and marked glacial oscillations. Minimum values during the Last Glacial Maximum (LGM) are consistent with colder and drier climate and presumably caused by the persistence of subtropical anticyclones over southern Africa limiting the amount of rainfall and vegetation growth. The otherwise higher glacial n-alkanes suggest a stronger influence of the tropical rainfall in southern Africa, likely associated with a contraction of the polar vortex with respect to its LGM position. During northern Heinrich events, moderate decline of n-alkanes suggests reduced rainfall over southern Africa possibly caused by weaker tropical easterly winds when, according to Stott et al. (2002), the Pacific Ocean would have experienced Super-ENSO conditions

Linking Ocean Variabilty to Phytoplankton Changes Over the Last 2000 Years off N. Iceland

International audienceUnderstanding ocean variability at decadal to century time-scale is a key issue of climate research, yet it remains poorly described partly because high resolution marine records are lacking. Recent observations also suggested that climate-driven oceanic changes may also affect marine productivity reflecting adjustments of the pelagic ecosystem. To explore these issues we investigated sediment cores (a Calypso and a box core) off North Iceland at unprecedented high temporal resolution (2 to 5 years). The area lies in a sensitive boundary region with strong climatic gradients between the Arctic and the North Atlantic realms. Surface hydrology is affected by warm, high salinity Atlantic water (Irminger current-IC) and cold, low salinity water of the East Icelandic Current (EIC), as well as by sea ice and drifting ice exported from the Arctic Ocean and East Greenland. Using biomarker proxies we reconstructed, over the past two millennia, high frequency surface ocean properties, such as Sea Surface Temperature (SST) and patterns of major phytoplankton groups. Records from a box core covering the last 80 years and comparison with instrumental data allowed to improve constrain of the proxies used. The SST curve reveals 4 major oscillations of about 20-25 years, depicting also the Great Salinity Anomaly (GSA). It reflects ocean circulation changes with varying impact of Polar and Atlantic waters (EIC, IC). Similarities with the NAO (North Atlantic Oscillation) index suggest links between STT and westerlies intensity. Phytoplankton biomarker profiles (specific phytosterols and alkenones) are consistent with biological data acquired during annual surveys. They show significant increase in periods when Atlantic waters and subsequent mixing processes prevail during positive NAO phase, compared to low levels when Polar waters and stratified conditions occur. Over the last two millennia the SST curve depicts a broad cooling trend towards present, steepening over the last 500 years. Prolonged warm or cool centennial intervals are recognizable among which the Medieval Warm Period (MWP), the Little Ice Age (LIA), the Roman Warm Period (RWP) and cooler Dark Ages. The SST signal also shows intermittent 20-25 year oscillations that are likely reflecting the ocean response to wind forcing, presumably the NAO. Phytoplankton biomarkers show important high frequency oscillations. Diatom sterols infer a broad decrease of siliceous productivity since 2000 yr BP with minimum values during the LIA, and a sharp rise after 200 yr BP. Such a trend is not observed for coccolithophorid biomarkers (alkenones), which show low centennial scale variations since 1200 yrs BP, along with high abundance between 1600 and 1200 yrs BP. Similar trends are observed for dinoflagellate biomarkers (dinosterol). Overall, planktonic biomarkers show multidecadal to centennial scale fluctuations inferring modifications of phytoplankton community structure. These could be linked at least partly to hydrological changes indicated in the SST curve, namely modifications of the relative importance of IC and EIC, which likely reflect the ocean response to NAO forcing

Rapid climate changes (RCCs) in the North-Western Mediterranean Sea during the Holocene

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The Baltic Sea inflow regime at the termination of the Medieval Climate Anomaly linked to North Atlantic circulation

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Mid-latitude Southern Indian Ocean response to Northern Hemisphere Heinrich events

International audienceThe lack of temporal resolution and accurate chronology of Southern Ocean marine cores has hampered comparison of glacial millennial-scale oscillations between the Southern Ocean, Antarctic ice and other records from both hemispheres. In this study, glacial climate variability is investigated over the last 50 ka using a multi-proxy approach. A precise chrono-stratigraphy was developed on the high-sedimentation rate core MD94-103 (Indian Southern Ocean, 45°35′S 86°31′E, 3560 m water depth) by geomagnetic synchronization between the later core and NAPIS75, and 14C dates. High-resolution time-series of δ18O in planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma, and sea surface temperatures (SSTs) estimated from the alkenone UK′37 index and foraminifera assemblages have been generated. Temporal evolution of the two temperature proxy records is notably different during the last glacial period. While foraminifera data indicate a consistent cooling towards the last glacial maximum, anomalous warm glacial alkenone temperatures suggest a strong advection of 〈warm〉 “detrital” alkenones by surface waters of the Agulhas current. Superimposed to this general trend, during Heinrich events, foraminiferal SSTs point to warmer surface waters, while concurrent alkenone SSTs exhibit apparent coolings probably caused by enhanced local alkenone production. By analogy to modern observations, possible influence of ENSO-like conditions on the subantarctic Southern Ocean SSTs is discussed

Atlantic gyres variability during the last millennium

International audienceWe investigate the low frequency variability of the Atlantic subpolar and subtropical gyres over the last millennium. First, a compilation of the several recent proxy reconstructions (e.g. Sicre et al. 2008, Richter et al. 2009 for the subpolar gyre, Mc Gregor et al. 2007 and unpublished data from Sicre et al. in the subtropical gyre) will allow to assess the low frequency hydrographic variability in key areas related to the horizontal circulation in the Atlantic and the upper limb of the Atlantic meridional overturning circulation. Second, we use a simulation of the IPSL model to explore the link between the gyres circulation and the local hydrography. In a simulation reproducing the climate over the last millennium, we assess the low frequency variability of the gyres circulation over this period and the role of the external forcing and low frequency atmospheric variability in the northern North Atlantic. The aim is to help the interpretation of the data cited above at the basin-scale

Sea surface temperature and sea ice variability in the subpolar North Atlantic from explosive volcanism of the late thirteenth century

International audiencethis study, we use IP25 and alkenone biomarker proxies to document the subdecadal variations of sea ice and sea surface temperature in the subpolar North Atlantic induced by the decadally paced explosive tropical volcanic eruptions of the second half of the thirteenth century. The short- and long-term evolutions of both variables were investigated by cross analysis with a simulation of the IPSL-CM5A LR model. Our results show short-term ocean cooling and sea ice expansion in response to each volcanic eruption. They also highlight that the long response time of the ocean leads to cumulative surface cooling and subsurface heat buildup due to sea ice capping. As volcanic forcing relaxes, the surface ocean rapidly warms, likely amplified by subsurface heat, and remains almost ice free for several decades

Sea surface temperature variability of the Labrador Current over the last 2000 years

International audienceThis study presents the first sub-decadal scale sea-surface temperature (SSTs) time-series derived from alkenone paleothermometry, covering the last 2000-year ocean temperature history of the the Labrador Sea region. The records obtained from two sites off Newfoundland document SST variations in a climatically crucial component of the Western North Atlantic circulation system, the southernmost Labrador Current (LC). This boundary current is a major conduit of cold and ice loaded fresh waters originating from the Arctic, which has a major impact on climate in the entire North Atlantic region. Our results demonstrate a clear link between the LC strength and the Northern Annular Mode (NAM), supporting the idea of a more persistent +NAM system and stronger LC during the Medieval Climate Anomaly (MCA). They also suggest enhanced LC activity under future warming with major implications for global thermohaline circulation