Fabien Dewilde

Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation.

The Southern Ocean plays a prominent role in the Earths climate and carbon cycle. Changes in the Southern Ocean circulation may have regulated the release of CO₂ to the atmosphere from a deep-ocean reservoir during the last deglaciation. However, the path and exact timing of this deglacial CO₂ release are still under debate. Here we present measurements of deglacial surface reservoir ¹⁴C age changes in the eastern Pacific sector of the Southern Ocean, obtained by ¹⁴C dating of tephra deposited over the marine and terrestrial regions. These results, along with records of foraminifera benthic-planktic ¹⁴C age and δ¹³C difference, provide evidence for three periods of enhanced upwelling in the Southern Ocean during the last deglaciation, supporting the hypothesis that Southern Ocean upwelling contributed to the deglacial rise in atmospheric CO₂. These independently dated marine records suggest synchronous changes in the Southern Ocean circulation and Antarctic climate during the last deglaciation.

Further constraints on the diagenetic influences and salinity effect on Globigerinoides ruber (white) Mg/Ca thermometry: Implications in the Mediterranean Sea

We analyzed Mg/Ca ratios of the planktonic species Globigerinoides ruber (white) picked from 49 box core samples covering the whole Mediterranean Sea and 2 core tops from the Atlantic Ocean. Over the entire data set, we found no significant correlation between Mg/Ca and δ18O-derived calcification temperatures. This lack of correlation is chiefly due to the presence of an early diagenetic, Mg-rich calcite coating, which can constitute up to 20% of the total shell calcite in the central and eastern Mediterranean basin and result in anomalously high Mg/Ca values and a high scattering. In the western Mediterranean Sea, however, G. ruber Mg/Ca scattering shows smaller amplitude and Mg-rich calcite remains under the XRD detection limit. SEM observations indicate that only a few samples are affected by trace amounts of post-mortem calcite overgrowths (most of this calcite being likely removed during the chemical cleaning for Mg/Ca analyses). Using core top sediments from the western Mediterranean Sea, we performed an empirical calibration exercise, which confirms that G. ruber Mg/Ca is not only related to temperature but it is also significantly affected by sea surface salinity. This salinity effect is not specific to high salinity environments such as the Mediterranean Sea, since it appears to be coherent with recent results obtained on Indo-Pacific and Atlantic surface sediments, which suggest that a +1 (psu) change in SSS results in a +1.7°C Mg/Ca-temperature bias. This sensitivity to salinity is significantly higher than those deduced from culture experiments.

Isolating climatic and paleomagnetic imbricated signals in two marine cores using principal component analysis

High resolution measurements of climatic and magnetic parameters have been performed on two cores from the eastern China Sea and the western Caroline Basin. On both cores, magnetic parameters show a strong imprint of climatic changes but the absence of relationship between the inclination and the bulk density indicates that the directional changes do not depend on lithology. A weak 100 ka cycle is present in the China sea inclination variations, but this period is not in phase with the orbital eccentricity and thus not relevant. All normalization parameters yielded similar estimates of relative paleointensity (RPI), but we have noticed the persistence of climatic components in the signal. Principal Component Analysis (PCA) applied to different parameters related to climate, lithology and paleointensity has allowed to extract a “clean” magnetic signal that we refer as “principal component of paleointensity (PCP)” which is in better agreement with the Sint-2000 composite curve and provides a reliable record of relative paleointensity. The presence of climatic frequencies in RPIs most likely reflects the influence of lithology on the response of magnetization to field intensity. We suggest that PCA analysis can be very useful to approach these problems. Not only can the calculation separate overlapping climatic and magnetic signals, but it indicates what confidence should be given to the data. Incidentally, the present results emphasize the importance of carrying out detailed paleoclimatic analyses along with paleointensity studies.

Comparison of 240 ka long organic carbon and carbonate records along a depth transect in the Timor Sea: Primary signals versus preservation changes

Total organic carbon (TOC) and calcium carbonate (CaCO3) records from different water depths in the Timor Sea (NE Indian Ocean) are compared in order to better reconstruct past changes in pelagic productivity, highlight the impact of preservation at depth, and unravel the interplay of organic carbon and carbonate sedimentation. New data are presented for core MD01-2376 located at 2376 m depth. These results are compared, over the last 240 ka, with published data on two neighboring cores (MD01-2378, 1783 m and MD98-2166, 3875 m). TOC fluctuations show strong glacial/interglacial variations and Milankovitch-type oscillations (i.e., dominant frequencies centered on the 100, 41, and 23 ka bands), with an enrichment during cold periods that reflects an increase in primary productivity as well as a lesser decomposition of organic matter with depth. During interglacials, TOC values are lower and show a stronger gradient with depth of deposition than during glacials. This suggests an overall better ventilation of deepwater masses and a steeper vertical gradient in the oxygen content, possibly associated to the upward extension of the oxygen-rich deep waters relative to the glacial situation. Temporal fluctuations in the CaCO3 records do not show such a glacial-interglacial pattern but reveal precession- and obliquity-related oscillations. Fluctuations in the 19–23 ka bands are only observed in the shallowest core, while fluctuations in the 41 ka band are clearly expressed in the two deepest cores, suggesting that this signal is more directly related to changes in carbonate preservation at the seafloor.