Marie-Alexandrine Sicre

Global temperature calibration of the alkenone unsaturation index (UK′37) in surface waters and comparison with surface sediments

In this paper, we compile the current surface seawater C37 alkenone unsaturation (UK′37) measurements (n = 629, −1 to 30°C temperature range) to derive a global, field-based calibration of UK′37 with alkenone production temperature. A single nonlinear “global” surface water calibration of UK′37 accurately predicts alkenone production temperatures over the diversity of modern-day oceanic environments and alkenone-synthesizing populations (T = −0.957 + 54.293(UK′37) − 52.894(UK′37)2 + 28.321(UK′37)3, r2 = 0.97, n = 567). The mean standard error of estimation is 1.2°C with insignificant bias in estimated production temperature among the different ocean regions sampled. An exception to these trends is regions characterized by strong lateral advection and extreme productivity and temperature gradients (e.g., the Brazil-Malvinas Confluence). In contrast to the surface water data, the calibration of UK′37 in surface sediments with overlying annual mean sea surface temperature (AnnO) is best fit by a linear model (AnnO = 29.876(UK′37) − 1.334, r2 = 0.97, n = 592). The standard error of estimation (1.1°C) is similar to that of the surface water production calibration, but a higher degree of bias is observed among the regional data sets. The sediment calibration differs significantly from the surface water calibration. UK′37 in surface sediments is consistently higher than that predicted from AnnO and the surface water production temperature calibration, and the magnitude of the offset increases as the surface water AnnO decreases. We apply the global production temperature calibration to the coretop UK′37 data to estimate the coretop alkenone integrated production temperature (coretop IPT) and compare this with the overlying annual mean sea surface temperature (AnnO). We use simple models to explore the possible causes of the deviation observed between the coretop temperature signal, as estimated by UK′37, and AnnO. Our results indicate that the deviation can best be explained if seasonality in production and/or thermocline production as well as differential degradation of 37:3 and 37:2 alkenones both affect the sedimentary alkenone signal.

Evaluation of long-chain alkenones as paleo-temperature indicators in the Mediterranean Sea

Long-chain alkenones were analyzed in samples collected from February to July 1989 by a time-series sediment trap located at 200 m depth in the northwestern Mediterranean Sea. The alkenone temperature signal was calibrated using water column particles collected at the sediment trap mooring site. The calibration (U37K′ = 0.041T−0.21, r2 = 0.97, 13°C < T < 19°C), differed from the culture calibration of Prahl et al. (1988, Geochimica et Cosmochimica Acta, 52, 2303–2310) and the calibration for the eastern North Atlantic waters between 16°C and 25°C (Cenre and Eglinton, 1993, Deep-Sea Research 1, 40, 1935–1961). The slope of the calibration was, however, similar to the slope of the calibration established using all Pacific Ocean samples (Sikes and Volkman, 1993, Geochimica et Cosmochimica Acta, 57, 1883–1889). The correlation of U38MeK and water temperature was also significant (U38MeK =0.042T−0.25, r2=0.87). Sediment trap results revealed that the production of alkenones during high flux periods predominantly occurred in subsurface waters. This was further supported by the depth distribution of alkenones observed during the monthly cruises: C37 alkenone concentrations in the upper 100 m of the water column indicated a subsurface maximum at approximately 50 m depth in May, June and July, whereas in March and November alkenones were more abundant in the upper 50 m. These results emphasize that accurate reconstruction of SST in core-tops must take into account the seasonality and depth variations of alkenone production. An intercalibration exercise, conducted on Mediterranean and Norwegian Sea water samples, demonstrated that high resolution chromatographic methods are essential to obtain reliable values of U37K′ and avoid the overestimation of production temperatures, particularly for colder waters where alkyl alkenoates are abundant.

Evolutionary trends in the lipid biomarker approach for investigating the biogeochemistry of organic matter in the marine environment

Abstract The detailed investigation of organic carbon cycling in estuarine and marine environments has stimulated the development of multidisciplinary concepts, research and sampling strategies as well as analytical tools in the last 10 years. Among other approaches, the molecular biomarker one has also undergone significant improvements. Some examples dealing with specific markers including sterols, alkenes and fatty acids are discussed in this paper in terms of source identification and transformation processes. Although some limitations exist for the elucidation of their stereochemistry, sterols have been used extensively to study biochemical processes affecting the organic matter in the water column and at the ocean/sediment interface. Sterols also appear as promising tracers of terrestrial vs. marine inputs in complex estuarine systems. The specificity of individual fatty acids or groups of acids has also been used to assess the origins and transformation processes of organic matter in marine samples. New approaches are presented including the investigation of intact lipid classes and the elucidation of the position of double bonds of unsaturated fatty acids and alkenes. Finally, we discuss the use of che-mometric techniques due to increasing chemical information and the interest of combining molecular-level and stable isotope approaches.

The glacial ocean productivity hypothesis: the importance of regional temporal and spatial studies

Abstract Higher ocean productivity has often been proposed to explain lower atmospheric carbon dioxide during the last glacial episodes. But recent consideration of marine cores from different areas show that higher local productivity can be postulated for interglacials as well as for glacial periods. Based on the detailed study of two piston cores from the northwest Africa upwelling system, the results presented here, including δ18O stratigraphy, organic carbon contents and fluxes, Ti Al ratios and grain size measurements, clearly indicate that the two cases of sedimentary records can even co-exist within a single upwelling system. This regional heterogeneity is presumably attributed to combined wind stress and sea-level changes that would induce different sedimentary records in the northern and in the southern part of the system. These results emphasize the importance to understand and to model the response of the main kinds of highly productive oceanographic regional systems which are spatially heterogeneous due to complex continent-ocean interactions, or to the presence of mobile hydrodynamic heterogeneities. For such an understanding it is not necessary to acquire a huge amount of core data throughout the world ocean, but to increase the density of cores as well as the regional-scale modelling efforts in systems such as coastal and equatorial upwelling areas, and the migration areas of the southern polar front.

The importance of terrestrial organic carbon inputs on Kara Sea shelves as revealed by n-alkanes, OC and δ13C values

Abstract Organic carbon (OC) and n -alkanes were investigated in the suspended matter (SM) and surficial sediments of the Arctic Ob and Yenisei Rivers and the adjacent Kara Sea. Organic nitrogen and δ 13 C values were determined as additional parameters in the sediments. OC and n -alkane concentrations were highly variable in space. This variability was interpreted as the result of a complex sedimentation pattern involving the discharges of these two rivers and the seasonal occurrence of a shore-ice front along the coast. Using the constant ratio of high molecular weight odd n -alkanes to OC in both rivers, we estimated that more than 70% of the OC preserved in shelf sediments is terrestrially-derived. This result was substantiated by the overall dominance of land-derived n -alkanes. A second approach using δ 13 C values and assuming binary dilution of riverine and marine OC led to comparable estimates.

A Review of 2000 Years of Paleoclimatic Evidence in the Mediterranean

The integration of climate information from instrumental data and documentary and natural archives; evidence of past human activity derived from historical, paleoecological, and archaeological records; and new climate modeling techniques promises major breakthroughs for our understanding of climate sensitivity, ecological processes, environmental response, and human impact. In this chapter, we review the availability and potential of instrumental data, less well-known written records, and terrestrial and marine natural proxy archives for climate in the Mediterranean region over the last 2000 years. We highlight the need to integrate these different proxy archives and the importance for multiproxy studies of disentangling complex relationships among climate, sea-level changes, fire, vegetation, and forests, as well as land use and other human impacts. Focusing on dating uncertainties, we address seasonality effects and other uncertainties in the different proxy records. We describe known and anticipated challenges posed by integrating multiple diverse proxies in high-resolution climate-variation reconstructions, including proxy limitations to robust reconstruction of the natural range of climate variability and problems specific to temporal scales from interannual to multicentennial. Finally, we highlight the potential of paleo models to contribute to climate reconstructions in the Mediterranean, by narrowing the range of climate-sensitivity estimates and by assimilating multiple proxies.

Refractory organic matter in sediments from the North–West African upwelling system: abundance, chemical structure and origin

Abstract Refractory (insoluble and nonhydrolyzable) organic matter (ROM) was studied in sediments underlying the north–west African upwelling system. Two core samples: beginning of glacial isotope stage 4 (ca. 70,000 yr BP; depth bsf 360 cm) and interglacial isotope substage 5d (ca. 90,000 yr BP; depth bsf 480 cm) were analyzed, along with a surface sample. ROM, which accounts for a substantial part of the total organic matter (OM) in these sediments (ca. 20% for the two core samples and 10% for the surface sample), was isolated and examined for its morphological and chemical features using a combination of transmission and scanning electron microscopy, spectroscopy (FTIR, solid state 13 C NMR) and analytical pyrolysis (Curie point flash pyrolysis–GC–MS). These studies allowed the chemical structure of the three ROMs (chiefly based on melanoidin-type macromolecules) and their mechanism of formation (degradation–recondensation of products mainly derived from proteinaceous material) to be established. Such a formation probably began in the water column and continued within the sediment upper layers. Important differences, concerned both with OM chemical structure and preservation mechanism, were noted upon comparison with recently studied sediments underlying the Peru upwelling system. These differences between the two systems, in spite of similarities in primary production, must reflect the combined influence of factors such as upwelling intensity (primary productivity), water depth and iron supply.

Biomarker stratigraphic records over the last 150 kyears off the NW African coast at 25°N

Terrigenous and marine biomarkers were investigated in a core oA Northwest Africa in the Northeast Atlantic (25N, 16W, 1445 m depth) to assess changes in the sedimentation pattern of organic carbon (OC) over the last 150 kyears. Alkenone derived temperatures recorded a warming of 4.5C during the last deglaciation. n-Alkanol Mass Accumulation Rates (MAR) fluctuated in parallel with Northeast Trade Winds (NETW) intensity. OC and sterol MAR both increased during glacial times indicating enhanced fertility of the ocean in response to intensified NETW. Alkenone/OC ratios were higher by a factor of two over stages 4‐6 than stages 1‐3 thus reflecting distinct coccolithophorid inputs. This transition coincides with a major change of alkenone producers inferred from coccolith counts. # 2000 Elsevier Science Ltd. All rights reserved.

Production pattern of alkenones in the Mediterranean Sea

Time-series of downward alkenone fluxes have been investigated at 200 m depth over a one year sediment trap experiment, in the Northwestern Mediterranean Sea. Alkenone flux maxima occurred in autumn and to a lesser extent in May, during the spring bloom. Temperature estimates calculated from the UK′37 index revealed that alkenone producers preferentially develop in subsurface waters (at about 50 m) in spring, whereas the autumn alkenone production occurred upper in the water column (around 30 m). Examination of the core-top UK′37 index values at various sites of the Northwestern Mediterranean basin, suggested that the spring bloom period do not significantly imprint the temperatures recorded in the sediments. The sedimentary temperature estimates would rather reflect annually integrated SST, with a major influence of the autumnal post-bloom development of the coccolithophores in the euphotic zone.

Hydrological changes in the Mediterranean Sea over the last 30,000 years

Sea surface temperatures were reconstructed over the last 30,000 years from alkenone paleothermometry (SST-alk) and planktonic foraminifera assemblages using the Modern Analog Technique (MAT) (SST-foram) along two cores of the Mediterranean Sea: MD84-632 (Levantine basin) and MD04-2797 (Siculo-Tunisian Strait). Oxygen isotope of planktonic foraminifera G. bulloides for core MD04-2797 and G. ruber for core MD84-632 were also determined. SST-alk in the Levantine basin indicate colder values at the Last Glacial Maximum (LGM) (∼14°C) than earlier established from MAT, and a cooling amplitude of 6°–7°C, comparable to the central Mediterranean Sea. Climatic events such as the Younger Dryas (YD) and Heinrich events 1 and 2 (H1 and H2) were times of significant cooling in the two cores. In the Eastern basin, values of local seawater oxygen isotope, δw, indicate relatively saltier waters during the LGM and deglaciation than today, with increasing δw values (higher salinity) in the Eastern basin and decreasing ones (lower salinity) in the central Mediterranean Sea, during cold stadials. The observed alterations of surface water properties (T and δw) in the central and eastern Mediterranean at the LGM are consistent with model experiments showing slightly lower evaporation in the Mediterranean than today, except for the Eastern basin.