Particle shuttling and oxidation capacity of sedimentary organic carbon on the Baltic Sea system scale

Madeleine M Nilsson,Astrid Hylén,Nils Ekeroth,Mikhail Y Kononets,Lena Viktorsson,Elin Almroth-Rosell,Per Roos,Anders Tengberg,Per O.J Hall

doi:10.1016/j.marchem.2021.103963

Abstract

Continental margin sediments receive most of the particulate organic carbon (POC) deposited on the global seafloor, making them crucial locations in the carbon cycle. However, the complex environments in coastal oceans make it challenging to predict the fate of sedimentary organic carbon (OC) in these areas. Here we use data from 21 sites in the Baltic Sea, representing different biological and physiochemical regimes, to explore controls on sedimentary OC cycling. To this end, we combine in situ measured benthic fluxes of dissolved inorganic carbon (DIC; proxy for OC oxidation) with data on sediment properties. In the Gulf of Bothnia, low sedimentary OC oxidation capacities (yearly DIC flux divided by sedimentary POC inventory) were likely caused by a large fraction of terrestrial material in the POC pool, indicated by low sedimentary chlorophyll a content and high (> 10) carbon:nitrogen ratios. The highest OC oxidation capacities were measured at shallow, permanently oxic sites in the Baltic Proper, where bioturbation likely stimulates OC oxidation. The other sites in the Baltic Proper and all stations in the Gulf of Finland displayed increasing OC oxidation capacities with increasing normalised water depth (station depth divided by maximal depth in the basin). This pattern suggests that substantial quantities of POC are shuttled, through repeated cycles of resuspension-redeposition, from shallow erosion-transport (ET) areas to deep accumulation (A) areas. This interpretation was supported by decreasing sediment age and increasing sedimentary inventories of POC and chlorophyll a with normalised water depth. Our calculations indicate that particle shuttling redistributes almost half of the deposited export production from ET areas to A areas in the Baltic Proper, and that substantial amounts of terrestrial organic material are transported through particle shuttling to the deeper parts of the Gulf of Finland and Gulf of Bothnia. Depositional setting and POC origin can thus be central factors in predicting the distribution and fate of OC in coastal and shelf sediments.

Highlights

Coastal oceans play a paramount role in the global carbon (C) cycle (Bauer et al, 2013; Bianchi et al, 2018; Gruber, 2015)
Our calculations indicate that particle shuttling redistributes almost half of the deposited export production from ET areas to A areas in the Baltic Proper, and that substantial amounts of terrestrial organic material are transported through particle shuttling to the deeper parts of the Gulf of Finland and Gulf of Bothnia
Geochemical signals in the sediment The benthic dissolved inorganic carbon (DIC) flux increased with water depth

Summary

Introduction

Coastal oceans play a paramount role in the global carbon (C) cycle (Bauer et al, 2013; Bianchi et al, 2018; Gruber, 2015). Coastal and shelf areas are generally nutrient rich and support high rates of primary production, resulting in elevated export production and a high deposition rate of organic matter on the seafloor (Dunne et al, 2007). Since such a high fraction of marine POC is deposited on coastal and continental margin sediments, the fate of this POC (i.e. burial or recycling) is of importance for C cycling globally

Methods

Results

Conclusion