Barium in hemipelagic sediment of the northwest Pacific: Coupling with biogenic carbonate

Abstract

[1] Geochemical analyses were performed on three cores of hemipelagic sediment from the northwest Pacific Ocean (eastern edge of Shikoku Basin) dating from the last interglacial and glacial periods. The objective of this work is to delineate the relations between excess sedimentary barium [Ba(ex)] content and biogenic opal, carbonate, and organic carbon contents. Calculated Ba(ex) values show variable degrees of correlation with biogenic sediment fractions. The mass accumulation rate (MAR) of Ba(ex) covaries with the MARs of biogenic fractions, and in particular, with the MAR of biogenic carbonate (r(2) = 0.68) and TOC (R-2 = 0.50). Variable relations between barium and biogenic fractions in hemipelagic sediment provide detailed insight into the behavior of sedimentary barium. It appears that a significant proportion of the barium is affiliated with the carbonate fractions. Based on this strong correlation, carbonate dissolution rates of the last glacial to interglacial periods are estimated. Assuming that the MARs of carbonate and Ba(ex) covary, variation of the Ba/Ca ratio in sediments reflects the extent of postdepositional carbonate dissolution. The record of sedimentary Ba/Ca ratios exhibits striking difference between sediment of the glacial and interglacial periods, with a higher positive correlation during glacial intervals and a lower correlation during interglacial intervals; this pattern is the result of enhanced carbonate dissolution during interglacial times. Sedimentary Ba/Ca ratios may, therefore, be a useful tool for estimating the relative extent of carbonate dissolution. Ba(ex) curves and patterns in sedimentary Ba/Ca ratios resemble the established Pacific carbonate dissolution record (i.e., enhanced dissolution during interglacial times and reduced dissolution during glacial times) as suggested by previous studies. Variations in sedimentary Ba/Ca ratios strongly support an interpretation of pronounced glacial-interglacial fluctuation in carbonate dissolution in the northwest Pacific Ocean. An especially pronounced dissolution event is evident at 75 kyr during the transition interval form interglacial to glacial conditions. This suggests that Ba(ex) may be used as a proxy for evaluating carbonate dissolution in hemipelagic settings, but further study of the association between barium content and carbonate dissolution will be required to confirm this.