Rain ratio variation in the Tropical Ocean: Tests with surface sediments in the eastern equatorial Pacific

Abstract

The organic carbon to calcite flux ratio (rain ratio) has a profound effect on the preservation of carbonates in the deep sea and may influence atmospheric pCO2 over millennia. Unfortunately, the degree to which the rain ratio varies in the more productive regions of the oceans is not well determined with sediment trap data. The rain ratio in the upper ocean appears dominantly linked to diatom productivity, which is not necessarily directly linked to total production and may be regionally variable. However, ballasting and protection of organic carbon by calcareous particles in the deeps may limit ratio variability at the seafloor. Sediment trap data do not exist for the regional determination of rain ratios in key highly productive areas like the eastern equatorial Pacific (EEP). To overcome this, we turn to surface sediment composition and accumulation rates as a representation of modern ratio variation. We present 230 Thorium ( 230 Th)-normalized carbonate, opal, organic carbon and detrital matter accumulation rates from core top samples in the EEP. We demonstrate a novel approach for estimating modern rain ratios from sedimentary proxies by (1) calculating vertical calcite flux from 230 Th-normalized carbonate accumulation rates (CARs) with correction for preservation and (2) calculating organic carbon fluxes with multiple algorithms that depend in varying degrees on ballasting. We find that organic carbon flux estimates from algorithms with and without a ballasting function produce results different from one another. Sediment accumulation rates for opal reflect the likely pattern of diatom production. By contrast, the organic carbon accumulation rate does not correlate well with surface ocean productivity or any of our algorithm-based organic carbon flux estimates. Instead, it correlates with the detrital component of the sediments suggesting an allochthonous input to sedimentary organic carbon accumulation in the EEP, which reduces its value as a productivity tracer. However, our calcite and multiple, satellite-based organic carbon fluxes allow estimation of the rain ratio and demonstrate a common regional pattern with moderate to strong variability in the rain ratio across the EEP. This variability is significant and is transmitted into the deeps leaving a sedimentary record regardless of the algorithm chosen to calculate organic carbon fluxes. Furthermore, we provide evidence suggesting that the rain ratio in the EEP may be driven by wind-supplied iron availability, which would regionally enhance nutrient use and promote diatom growth. r 2007 Elsevier Ltd. All rights reserved.