Productivity and sediment focusing in the Eastern Equatorial Pacific during the last 30,000 years

Abstract

The Eastern Equatorial Pacific (EEP) affects the ocean-atmosphere exchange of CO2 on seasonal and interannual time scales through a balance of upwelling of CO2-rich waters and the drawdown of CO2 by biological productivity in the surface waters. The EEP accounts for almost 3/4ths of the global oceanic outgassing of CO2 to the atmosphere, and it is known that the size of this EEP source of CO2 varies significantly during El Niño events (Feely et al., 1999). There has been much effort to determine the El Niño Southern Oscillation (ENSO) state of the Equatorial Pacific during the past, particularly at the Last Glacial Maximum (LGM) when the global atmospheric [CO2] was low, yet the glacial ENSO state remains a source of considerable controversy (Ford et al., 2015; Herguera, 2000; Koutavas et al., 2002; Loubere et al., 2004; Lyle, 1988; Paytan et al., 1996; Pedersen, 1983; Sarnthein et al., 1988). Reconstructing past changes in equatorial productivity could help establish the prevailing ENSO state of the Pacific during the LGM, as the El Niño-related deepening of the thermocline in the East Pacific reduces productivity in the EEP and increases it in the Western Equatorial Pacific. Here we investigate changes in productivity in four cores from the equatorial Pacific, in the heart of the modern equatorial cold tongue. We determine changes in productivity using measurements of 231Pa, 230Th, 232Th, and 238U along with sedimentary fluxes. We also compare our findings to other sediment cores in the Pacific. We find elevated (231Pa/230Th)xs values (higher than production values) in general across the cores, indicating a net sink for oceanic 231Pa in the EEP. We also find evidence for low levels of lateral sediment focusing, as well as lower productivity during the glacial in reduced 230Th-normalized opal fluxes and decreased (231Pa/230Th)xs at multiple sites. Examination of authigenic uranium at our sites in conjunction with previous work (Jacobel et al., 2017) shows that between 2 and 3.5 km depth in the Equatorial Pacific, there was a floating pool of respired carbon associated with the southward return flow of North Pacific Deep Water, sequestering CO2 from the atmosphere during the LGM. We also compile Pacific basin wide records of productivity and Pa/Th during the Holocene (0-11kya) and LGM (18-22kya) and find evidence consistent with a more frequent or persistent glacial El Niño state throughout much of the Pacific (North Pacific, Western Equatorial Pacific and EEP).