Abstract
Abstract. We present a one-dimensional reactive transport model to estimate benthic fluxes of dissolved inorganic carbon (DIC) and alkalinity (AT) from coastal marine sediments. The model incorporates the transport processes of sediment accumulation, molecular diffusion, bioturbation and bioirrigation, while the reactions included are the redox pathways of organic carbon oxidation, re-oxidation of reduced nitrogen, iron and sulfur compounds, pore water acid-base equilibria, and dissolution of particulate inorganic carbon (calcite, aragonite, and Mg-calcite). The coastal zone is divided into four environmental units with different particulate inorganic carbon (PIC) and particulate organic carbon (POC) fluxes: reefs, banks and bays, carbonate shelves and non-carbonate shelves. Model results are analyzed separately for each environment and then scaled up to the whole coastal ocean. The model-derived estimate for the present-day global coastal benthic DIC efflux is 126 Tmol yr−1, based on a global coastal reactive POC depositional flux of 117 Tmol yr−1. The POC decomposition leads to a carbonate dissolution from shallow marine sediments of 7 Tmol yr−1 (on the order of 0.1 Pg C yr−1. Assuming complete re-oxidation of aqueous sulfide released from sediments, the effective net flux of alkalinity to the water column is 29 Teq. yr−1, primarily from PIC dissolution (46%) and ammonification (33%). Because our POC depositional flux falls in the high range of global values given in the literature, the reported DIC and alkalinity fluxes should be viewed as upper-bound estimates. Increasing coastal seawater DIC to what might be expected in year 2100 due to the uptake of anthropogenic CO2 increases PIC dissolution by 2.3 Tmol yr−1and alkalinity efflux by 4.8 Teq. yr−1. Our reactive transport modeling approach not only yields global estimates of benthic DIC, alkalinity and nutrient fluxes under variable scenarios of ocean productivity and chemistry, but also provides insights into the underlying processes.
Highlights
Based on globally averaged carbon box models driven by atmospheric CO2, globally averaged land-use changes and climate, Andersson et al (2005) recently estimated that the coastal ocean currently receives ∼ 80 Tmol C yr−1 from terrestrial sources, pumps ∼ 20 Tmol C yr−1 from the atmosphere and sequesters on the order of 40 Tmol C yr−1 in shallow marine sediments
Values are either extracted from the literature or derived from the 1◦ dataset of the Levitus94 database (NOAA World Ocean Atlas 1994, available online at http://iridl.ldeo.columbia.edu/). This simplified approach can be seen as a first step towards a better regional classification of the coastal ocean in terms of the dominant controls on carbonate chemistry. It implies that our results only provide a first-order global estimate of dissolved inorganic carbon (DIC) and alkalinity fluxes for the coastal ocean
Because the DIC buried with pore waters is very small (< 1 %), and in situ CaCO3 precipitation and FeCO3 precipitation are predicted to be minimal, the benthic DIC efflux is essentially equal to the sum of the depth-integrated rates of particulate organic carbon (POC) oxidation and particulate inorganic carbon (PIC) dissolution
Summary
Based on globally averaged carbon box models driven by atmospheric CO2, globally averaged land-use changes and climate, Andersson et al (2005) recently estimated that the coastal ocean currently receives ∼ 80 Tmol (inorganic and organic) C yr−1 from terrestrial sources, pumps ∼ 20 Tmol C yr−1 from the atmosphere and sequesters on the order of 40 Tmol C yr−1 in shallow marine sediments. Andersson et al (2005) predict that increasing coastal productivity over the century will significantly alter the CO2 pumping efficiency of the global coastal ocean This eutrophication will increase POC deposition fluxes, and, because organic carbon metabolism is the major driving force for PIC dissolution in shallow environments (Morse and Mackenzie, 1990), benthic carbonate dissolution will increase. The purpose of this contribution is to determine the quantitative significance of major early diagenetic processes for the production and consumption of DIC and alkalinity, as well as their benthic fluxes in coastal ocean sediments, for both present-day conditions and in response to coastal ocean eutrophication and acidification scenarios To this end, a onedimensional (1-D) reactive transport model (RTM) of benthic organic carbon and inorganic carbon dynamics is presented. We explore the response of DIC and alkalinity fluxes to changes in organic and inorganic carbon deposition fluxes due to eutrophication and ocean acidification, respectively
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