River supply, ocean burial and the contemporary carbon cycle

The transfer of organic carbon across the land-ocean interface is constrained using river and marine sediment radiocarbon data and machine learning
Published in Earth & Environment
River supply, ocean burial and the contemporary carbon cycle
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The ocean represents the ultimate sink of particles and organic carbon, irrespective of whether they are produced by weathering on land, eroded, and transported to the sea, or produced by organisms in the ocean. Sediments accumulating at the seafloor bury organic carbon along and this organic carbon burial represents a sink of about 0.3 Pg C y-1 (1 Pg = 1015 gram) that is at par with the annual delivery of riverine particulate organic carbon (POC) to the ocean. If there would be a one-to-one correspondence between global river POC delivery and ocean POC burial, this carbon flow would be neutral to the global contemporary carbon cycle and its perturbation.

However, a major part of the terrestrial organic matter transported to the ocean is degraded and only a minor part is eventually buried, and most marine sediments are dominated by locally produced organic matter rather than terrestrial organic matter.  This implies that the nature of organic carbon buried differs from that delivered to the ocean. Moreover, rivers transport organic carbon from various sources: eroded from rocks and soils, exported by terrestrial and littoral ecosystems, and produced within inland waters. These different types of organic matter differ in composition, reactivity, degradation history and age and show distinct spatial patterns related to geological settings, dominant ecosystems and regional climate and hydrology. Similarly, the receiving ocean margin systems exhibit distinct biogeochemical characteristics due to differences in water depth, hydrodynamics, water residence times, primary production and ecosystem structure and functioning.

These spatial differences in riverine POC supply and ocean margin biogeochemistry cause that some ocean margins are a source of carbon dioxide to the atmosphere (river POC input is larger than burial), while others may act as a sink (POC burial is larger than regional supply of riverine POC). 

Radiocarbon constraints

Elucidating the global patterns of POC delivery to and burial in ocean margins requires a spatially resolved common proxy for the nature of organic carbon in riverine particles and marine sediments.  Radiocarbon has emerged as an excellent proxy and thanks to community efforts and open science, large datasets are now available for both riverine POC and marine surface sediments.  Combining these datasets would in principle allow extracting organic carbon transfer patterns across the land-ocean interface, regionally as well as globally. However, the spatial coverage of both the river and marine sediment datasets is rather limited and there are very few geographical areas with the adjacent river and marine sediment radiocarbon data needed to assess organic carbon transfer across the land-ocean continuum. 

We have combined the riverine and marine surface sediment radiocarbon data with an extensive dataset for environmental variables and used machine learning to generate the first high-resolution global map of radiocarbon values in the river-ocean continuum.

Radiocarbon values of organic carbon in river particulates and in marine surface sediments of the Amazon river-ocean continuum.

A high-resolution global map of radiocarbon of POC

This machine-learning generated radiocarbon map will be useful as a reference basis for future food-web and biogeochemistry studies, and displays systematic and distinct latitudinal patterns for both river and surface marine radiocarbon values. Crucially, it reveals accumulation hotspots for modern and aged organic carbon in surface marine sediments and their relations with the nature of river POC. Arctic shelves receiving permafrost carbon and mountainous rivers discharging onto narrow shelves are examples of systems with old riverine and marine sediment organic carbon. The Amazon and Mississippi are examples of rivers delivering relatively young organic carbon to the ocean and having deltaic sediment with old organic carbon: these act as a source of carbon dioxide to the atmosphere. The coast of South Africa is an example of a carbon sink where old riverine carbon is delivered but young, locally produced material is accumulating in marine sediments. This global radiocarbon map will be useful for the Earth System modelling community to constrain the regional river carbon input to the ocean needed to accurately quantify anthropogenic carbon inputs. It will also provide guidance to researchers to identify hotspots and areas where data collection should be prioritized. Ocean margin sediments represent an important carbon sink and managing this crucial carbon sink requires high-resolution information on the quantity and nature of carbon accumulating. Our high-resolution carbon isotope maps will be instrumental for informed decisions on regional and national blue carbon storage.

For more detailed information and access to the code and data, please see https://doi.org/10.1038/s41561-024-01476-4

The article was published in Nature Geosciences on June 21, 2024:

Wang, C., Qiu, Y., Hao, Z., Wang, J., Zhang, C., Middelburg, J.J., Wang, Y., Zou, X. Global patterns of organic carbon transfer and accumulation across the land-ocean continuum constrained by radiocarbon data. Nature Geoscience, https://doi.org/10.1038/s41561-024-01476-4

Authors of the blog:

Jack J. Middelburg and Junjie Wang, Utrecht University

Chenlong Wang and Xinqing Zou, Nanjing University

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Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geochemistry > Biogeochemistry
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