Barium Concentrations as a Proxy for Equatorial Pacific Productivity during the Past 140,000 Years

Abstract

The accumulation of biogenic sediments in the deep ocean may yield important information about past ocean productivity and its relationship to climate change. Sediments, sampled at approximately millennial resolution, every 4 – 5 cm on average, were obtained from a core (ML1208-17PC) retrieved from the Central Equatorial Pacific (CEP) ocean (0.48°N,156.45°W; water depth 2,926 m). The main objectives of this research were to (1) use barium concentrations in marine sediments as a proxy for past ocean export production, and to (2) determine how past glacial-interglacial changes in productivity are related to climate and, specifically, variability of dust fluxes which, in turn, are a function of changing atmospheric circulation patterns. Ba concentrations on complete digestions of bulk sediment were measured by inductively-coupled plasma mass spectrometry. Sediments ranging in age from late Holocene to 140 kyr yielded bulk barium concentrations that ranged from about 305 to 670 ppm. The authigenically-derived portion of the total barium, thought to be correlated with export production, is denoted as the biogenically-derived barium (bioBa). Using accumulation rates derived by Jacobel et al. (2017), we found bioBa accumulation rates that ranged from about 0.5 to 1.0 mg cm-2 kyr-1. For this research, the following hypotheses were tested: 1.) BioBa fluxes (i.e., productivity) will be higher at the equatorial site, during the last glacial maximum (Marine oxygen Isotope stage 2 (MIS 2); i.e. 25 – 12 kyr), than the same fluxes during the MIS 1 (i.e., 12 – 0 kyr). 2.) BioBa fluxes (i.e. productivity) will be higher during glacial MIS 4 and MIS 6 than those during interglacial MIS 3 and MIS 5. Over the past 140 kyr, there is a positive relationship between productivity and warm interglacial periods which, in turn, are anti-correlated with dust fluxes determined by Jacobel et al. (2017). Thus, high bioBa accumulation rates correspond to less dusty warm periods, and vice versa. Productivity is apparently not a function of dust fertilization in the central equatorial Pacific, on Milankovitch glacial-interglacial timescales.