Benthic d13C and Cd/Ca Records of Mid-depth Ventilation from the Upper Portuguese Continental Margin

C. Willamowski GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany

J. Schönfeld GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany

R. Zahn GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany

We present benthic foraminiferal Cd/Ca and d13C records of core SO75-26KL from the southern Portuguese continental margin (37°49.3'N, 20°30.2'W, 1099 m water depth). Cd (and Mn) analyses were done using Uvigerina peregrina and Uvigerina mediterranea and d13C measurements were carried out on Cibicidoides wuellerstorfi, C. kullenbergi and C. pseudoungerianus. According to the planktonic and benthic oxygen isotope records this core reaches back to 50 ka. Three IRD events are recognised in the core which are coeval to "Heinrich" events 1, 2 and 4 in the open North Atlantic. The benthic d13C record shows high glacial values equal or higher than 1.5 ” and a late-glacial transition to lower values of 1 ” which are maintained during the Holocene. During the IRD events strong negative d13C anomalies exist.

The Cd/Ca- record is different in that it does not display a systematic offset between glacial and Holocene values; values remain around 0.03 µmol/mol. During the IRD-events the Cd/Ca-data shows distinct maxima. The Mn/Ca- ratios were determined to check for secondary calcite overgrowth which may potentially alter the Cd/Ca ratios. From the slightly elevated Mn/Ca ratios it is obvious that secondary Mn-enrichment does exist on the foram tests. However, all samples are still in a reliable range of Mn/Ca < 100 µmol/mol. Thus, even though secondary calcification as potentially evidenced by Mn-coatings can affect the Cd/Ca-ratios in foraminiferal tests, the Cd/Ca data from Core SO75-26KL, partly because of their good correlation with the benthic d13C data, look convincing enough to use them for paleogeochemical interpretation.

We interpret the discrepancy between the pronounced d13C drop but unchanged Cd/Ca levels during the last glacial-
interglacial transition as evidence for a thermodynamic overprint on benthic d13C. This overprint may have been caused by a shift in source water mass from cold North Atlantic waters during the last glacial to primarily Mediterranean waters during the Holocene. At each of the glacial IRD events d13C and Cd/Ca consistently indicate an increase in dissolved biologically-cycled constituents which points to abrupt short-lived reductions in water mass advection.