Biogenic budgets of particle rain, benthic remineralization and sediment accumulation in the equatorial Pacific

Abstract

Budgets of organic C ( C org), CaC0 3 and opal have been constructed for the Palisades, NY Pacific equatorial region at 140°W between 5°N and 5°S. Measurements of the rain and benthic remineralization rate of biogenic materials have been adjusted and normalized to account for sampling biases. Sea surface temperature serves as a master variable in normalizing sediment trap and benthic remineralization data to average conditions. The rain and remineralization rates for C org are nearly equal: 0.40±0.05 and 0.46±0.06 mmol m −2 d −1 respectively; thus only a minor fraction of this constituent is buried. Rain and dissolution rates for biogenic opal are similarly balanced (0.3±0.06 and 0.36±0.01 mmol m −2 d −1) and consistent with the value for opal burial (0.0±0.004). The CaC0 3 budget appears to have changed during the Holocene. The best estimates of modern CaC0 3 dissolution (0.58±0.03 mmol m −2 d −1) and rain rate (0.61±0.06) are consistent with 230Th-normalized carbonate accumulation rates for the late Holocene (0.1 mmol m −2 d −1). However, the balance between dissolution and rain is not consistent with early Holocene carbonate accumulation (0.3 mmol m −2 d −1 ), and this imbalance suggests: 1) a recent increase in the rate of CaC0 3 dissolution on the sea floor, or 2) a decrease in the rain rate of carbonate particles. Modeling 230Th profiles in sediments from this region define the last 3000 years as the duration of increased dissolution or decreased particle rain. 231Pa/ 230Th ratios in sediments indicate that particle rain rates have remained constant or possibly increased slightly through the Holocene. Two potential causes for increased dissolution were investigated; a change in deep water carbonate saturation or a change in Co,g/CaC0 3 rain ratios. A model describing carbonate dissolution as a function of the degree of undersaturation and the amount of organic carbon oxidation within sediments indicates that the recent increase in dissolution is more likely due to changes in bottom water chemical composition. We propose that Pacific Ocean bottom water carbonate ion concentration has decreased by 10–15 μM over the last 3000 years.