Perspectives on the Terrestrial Organic Matter Transport and Burial along the Land-Deep Sea Continuum: Caveats in Our Understanding of Biogeochemical Processes and Future Needs

Abstract

The natural carbon cycle is immensely intricate to fully understand its sources, fluxes and the processes that are responsible for their cycling in different reservoirs and their balances on a global scale. Anthropogenic perturbations add another dimension to such a complex cycle. Therefore, it is necessary to update the global carbon cycle by combining both natural and anthropogenic sources, fluxes and sinks along the land-sea continuum to assess whether these terms are currently in balance or not. Here, we review the export and burial rates of terrestrial organic carbon in the oceans to understand the issue of "missing terrigenous carbon" by comparing data-and model-based estimates of terrestrial carbon fluxes. Our review reveals large disparities between field data and model output in terms of dissolved and particulate organic carbon/matter (OC/OM) fluxes and their ratios, especially for Oceania and Arctic rivers, suggesting the need of additional investigations in these regions to refine terrestrial OC export budget. Based on our budgeting of global sources and sinks of OC with updated estimates of marine productivity and terrestrial OM burial rate, we find that the marginal sediments are key burial sites of terrestrial OM (TOM), which is consistent with earlier views of Berner (1982) and Hedges and Keil (1995). While about 60-80% of TOM is remineralized in the margins, the estimated budget further reveals the ocean derived OM is efficiently remineralized than that of terrestrial OM, emphasizing the need of further improvements of carbon burial estimation in the marine realm. When we look back in the past, higher terrestrial OC burial (by ∼50%) in the deep ocean during the glacials than during the interglacials and suggests the subdued role of continental margins and an efficient transfer and preservation of OM from the shelf to deep sea in glacials. Based on the review of terrestrial and marine OM burial, we suggest some critical regions/ways that need to be investigated/addressed further, identification of new biogeochemical proxies and their grouping to better constrain the global carbon cycle along the land-deep sea continuum in future.

Carbon on the Northwest European Shelf: Contemporary Budget and Future Influences

“…CO 2 emissions from European estuaries alone represent 5-10% of Western Europe's anthropogenic CO 2 emissions (Frankignoulle et al, 1998). Some terrigenous DOC (tDOC) is flocculated and buried within shelf and ocean sediments, contributing up to 30% to ocean sedimentary burial (Burdige, 2005;Kandasamy and Nagender Nath, 2016). More refractory tDOC may be transported far from the coast and even off-shelf.…”

Biogeochemical characteristics of suspended particulate matter in deep chlorophyll maximum layers in the southern East China Sea

“…However, in most cases, end-member values of δ 13 C, δ 15 N and molar C / N ratios are represented by "typical" numbers, such as ca. −20 and −27 ‰ for δ 13 C of marine phytoplankton and terrestrial plants (Kandasamy and Nagender Nath, 2016, and references therein), respectively, but without measuring discrete endmember values in real, local or regional OM source materials. For example, a number of earlier studies failed to measure isotopic values of marine phytoplankton despite using end-member mixing models to distinguish marine versus terrestrial OM in surface sediments (e.g.…”

“…For example, a number of earlier studies failed to measure isotopic values of marine phytoplankton despite using end-member mixing models to distinguish marine versus terrestrial OM in surface sediments (e.g. Kao et al, 2003;Wu et al, 2013), or these numbers were simply represented by values of particulate organic matter (POM) in surface waters in the studied system (e.g. Zhang et al, 2007) or elsewhere from other ocean basins (e.g.…”

Impact of Hydrological Conditions on the Biogeochemical Dynamics of Suspended Particulate Organic Matter in the Upper Mixed Layer of the Southern East China Sea

“…Globally, the oceans are an important repository of organic carbon, collecting~5.07 × 10 16 g C/year from marine primary production (Carr et al, 2006) and~4.19 × 10 14 g C/year from terrestrial sources mainly through rivers (Kandasamy & Nagender Nath, 2016;Schlünz & Schneider, 2000). The marginal seas located at the interface between land and open ocean receive particles and nutrients from these reservoirs, acting as key areas for biogeochemical cycles of carbon and other nutrients (Hickman et al, 2012).…”