The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean

Tim Rixen,Birgit Gaye,Kay-Christian Emeis,Venkitasubramani Ramaswamy

doi:10.5194/bg-16-485-2019

Abstract

Abstract. Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO2 uptake of the organic carbon pump in the receiving ocean areas.

Highlights

Photosynthesis and the export of organic matter from the euphotic zone into the deep sea drive the organic carbon pump and are integral parts of the global carbon cycle (Volk and Hoffert, 1985)
At WAST the analysis suggests that carbonate is the main ballast mineral and – in contrast to our previous inference – at Java Mooring (JAM), which is surprising considering the low carbonate and high lithogenic matter content in samples from these two sites (Tables 4, 5)
The evaluation of data from sediment trap time series in conjunction with satellite-derived observations suggests that at sites characterised by low lithogenic matter contents (< 25 %) primary production is the dominant control of the spatial variability of organic carbon fluxes into the deep ocean

Summary

Introduction

Photosynthesis and the export of organic matter from the euphotic zone into the deep sea drive the organic carbon pump and are integral parts of the global carbon cycle (Volk and Hoffert, 1985). The CO2 uptake in the ocean surface by phytoplankton and its transfer as sinking organic carbon through the thermocline and into the deep ocean is determined by available nutrients. The nutrients and CO2 associated with sinking organic matter are regenerated during passage through the water column and these “regenerated nutrients” comprise approximately 50 % of the ocean’s nutrient inventory. The ratio between total (regenerated and preformed) nutrient input into the ocean’s surface mixed layer and their export as organic matter across the thermocline is a measure of the CO2 uptake efficiency by the organic carbon pump (DeVries et al, 2012).

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Results

Conclusion