Orbital Variations of Biogenic CaCO3 and Opal Abundance in the Western and Central Equatorial Pacific Ocean During the Late Quaternary

Abstract

Biogenic CaCO 3 and opal abundances were measured in two piston cores (PC313 and PC5101) collected, respectively, along the equator in the western and central Pacific Ocean. The age model for core PC313, which extends to about 750 ka, was developed by comparing the oxygen isotope stratigraphy of planktonic foraminifera (N. dutetrei) to the SPECMAP stack. The age model for core PC5101, which extends to about 600 ka, was developed by stratigraphic correlation of CaCO 3 contents to those in the well-dated core RC11-210 (Chuey et al. 1987). Both cores distinctly exhibited a series of CaCO 3 and opal variations, which are mainly controlled by the orbital eccentricity cycle of about 100-kyr. The orbital-scale cyclic variations of CaCO 3 and opal contents appear to be contrasting in both cores such that high CaCO 3 and low opal contents occurred during the glacial periods. In contrast, during the interglacial periods, low CaCO 3 and high opal contents occurred. Mostly remarkable is the distinct occurrence of a mid-Bruhnes event (MBE) at around 350 ka. The CaCO 3 content was higher in core PC5101 than in core PC313 before the MBE, whereas biogenic opal abundance became higher in core PC5101 after the MBE. Such a characteristic discrepancy of biogenic (CaCO 3 and opal) production, i.e., a succession of primary producers from coccolithophore to diatom, between cores PC313 and PC5101 may be attributed to the prevailing dominant hydrographic conditions (i.e., the South Equatorial Current), in the path of which both cores are located. The intensity of westward propagation might have been an important factor in contrasting biogenic production centering around the MBE.

Biogenic CaCO3 and Opal Depositions and Their Latitudinal Comparison During the Past 600 ka in the Central Equatorial Pacific

“…Both cores display higher CaCO 3 contents during glacial periods than during interglacial periods, and vice versa for opal. Khim et al (2012) already reported on the similar geochemical properties in the western equatorial Pacific, which were compared regionally with those in the eastern equatorial Pacific (Murray et al 1995), showing consistent orbital-scale variation. The fluctuations in these biogenic components in both cores are greater during interglacial than during glacial periods (Fig.…”

“…Verardo and McIntyre (1994) reported on the glacial and interglacial cyclicity of CaCO 3 and opal concentrations and their MARs in the equatorial Atlantic, identifying late Pleistocene signals of regional paleoclimatic change. Similarly, a series of quasi-regular variations in biogenic CaCO 3 and opal content, which are mainly controlled by the orbital eccentricity cycle of approximately 100 ka, were clearly observed along the Equator in the equatorial Pacific (Khim et al 2012).…”

“…Rea et al (1991) reported that the CaCO 3 and opal contents of RC11-210 also show a prominent 100 ka spectral peak. Based on spectral analysis performed using standard menu-driven software applying the Blackman-Tukey method, Khim et al (2012) confirmed that the CaCO 3 and opal abundances of PC5101 are controlled primarily by eccentricity forcing.…”

“…The opal content was calculated by multiplying the biogenic silica content by 2.4 (Mortlock and Froelich 1989). The CaCO 3 and opal contents of PC5101 were previously reported by Khim et al (2012).…”

“…This CaCO 3 record serves as a reliable and detailed means of chemostratigraphic correlation (Berger and Vincent 1981). High-and low-CaCO 3 intervals develop a distinct sequence of events, which can be correlated over wide areas of the equatorial Pacific (e.g., Khim et al 2012). RC11-210 shows quasi-periodic cycles in CaCO 3 content (50 -97%; Fig.…”