BARIUM CONCENTRATIONS AS A PROXY FOR EQUATORIAL PACIFIC PRODUCTIVITY DURING THE PAST 140,000 YEARS

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, have been obtained from a core (ML1208-17PC) retrieved from the central equatorial Pacific Ocean (0.48°N,156.45°W, water depth 2,926 m). The main objective of this research has been to use concentrations of barium in the sediment record as a proxy for past ocean export production, and to determine how past changes in productivity are related to climate and, specifically, variability of dust fluxes on glacial-interglacial timescales.

Ba concentrations on complete digestions of bulk sediment have been measured by inductively-coupled plasma mass spectrometry. Sediments ranging in age from late Holocene to 130 kyr have yielded bulk barium concentrations that range 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 excess barium (xsBa). The xsBa is estimated by subtracting the lithogenic Ba from the total Ba. Here lithogenic Ba contents have been calculated using published thorium concentrations in the same samples (Jacobel et al., 2016). The xsBa makes up greater than 97% of the total Ba. Using accumulation rates derived by Jacobel et al. (2016), we find xsBa accumulation rates that range from about 0.5 to 1.0 mg cm^-2 kyr^-1. Over the past 120 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 (2016). Thus, high xsBa 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, and we discuss the implications of these findings.

Jacobel et al. (2016) Climate-related response of dust flux to the central equatorial Pacific over the past 150 kyr, Earth and Planetary Science Letters 457, 160-172.