Influence of volcanic shards on foraminiferal Mg/Ca in a core from the Galápagos region

Abstract

Variability in the Mg, Fe, Al, Mn, and U to Ca composition of the planktonic foraminifera G. ruber in core TR163-22, an 8.8 m long piston core from just northwest of the Galapagos Islands, reveals distinct patterns of downcore behavior. Mg/Ca data follow a general climatic trend, with lower values during cold intervals (i.e., marine isotope stages (MIS) 2-4) and higher values during warm intervals (i.e., MIS 1 and 5). Fe/Ca and Al/Ca data reveal distinct intervals in which concentrations are elevated tenfold or more over baseline values, with highly variable elemental ratios in these intervals. The Mg/Ca data are highly correlated with the Fe/Ca and Al/Ca data in these intervals, with slopes of 1.13 ±0.03 and 0.66 ±0.07 mole/mole, respectively. Examination of the sediments from these intervals reveals that they are rich in volcanic debris that is likely derived from Galápagos volcanic rocks. The presence of small amounts of this volcanic debris can bias shell Mg/Ca to elevated values. Attempts to use enhanced cleaning to remove the volcanic particles were largely unsuccessful. The most successful strategy is to use the Fe/Ca and Al/Ca from the shell digests along with the Fe to Mg ratio of the volcanic debris to correct the Mg/Ca data for Al-silicate contamination, in which case the corrected data more clearly reveal the primary climatological patterns. Mn/Ca and U/Ca data are characterized by cycles of 20-200 cm length, with Mn/Ca having a distinct trend toward higher values between the core top and 200 cm. These trends appear to be related to diagenetic coatings on the foraminifera shells, most likely in the form of Mn-carbonates. Copyright 2005 by the American Geophysical Union.