Assessing global-scale organic matter reactivity patterns in marine sediments using a lognormal reactive continuum model

Xu S., Liu B., Arndt S., Kasten S. and Wu Z.

Organic matter (OM) degradation in marine sediments is largely controlled by its reactivity and profoundly affects the global carbon cycle. Yet, there is currently no general framework that can constrain OM reactivity on a global scale. In this study, we propose a reactive continuum model based on a lognormal distribution (l-RCM), where OM reactivity is fully described by parameters μ (the mean reactivity of the initial OM bulk mixture) and σ (the variance of OM components around the mean reactivity). We use the l-RCM to inversely determine μ and σ at 123 sites across the global ocean. The results show that the apparent OM reactivity decreases with decreasing sedimentation rate (ω) and that OM reactivity is more than 3 orders of magnitude higher in shelf than in abyssal regions. Despite the general global trends, higher than expected OM reactivity is observed in certain ocean regions characterized by great water depth or pronounced oxygen minimum zones, such as the eastern–western coastal equatorial Pacific and the Arabian Sea, emphasizing the complex control of the depositional environment (e.g., OM flux, oxygen content in the water column) on benthic OM reactivity. Notably, the l-RCM can also highlight the variability in OM reactivity in these regions. Based on inverse modeling results in our dataset, we establish the significant statistical relationships between 〈k〉 and ω and further map the global OM reactivity distribution. The novelty of this study lies in its unifying view but also in contributing a new framework that allows predicting OM reactivity in data-poor areas based on readily available (or more easily obtainable) information. Such a framework is currently lacking and limits our abilities to constrain OM reactivity in global biogeochemical or Earth system models.

link to the article

Coastal vegetation and estuaries are collectively a greenhouse gas sink

Rosentreter, J.A., Laruelle, G.G., Bange, H.W. (...) and Regnier P.

Coastal ecosystems release or absorb carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), but the net effects of these ecosystems on the radiative balance remain unknown. We compiled a dataset of observations from 738 sites from studies published between 1975 and 2020 to quantify CO₂, CH₄ and N₂O fluxes in estuaries and coastal vegetation in ten global regions. We show that the CO₂-equivalent (CO₂e) uptake by coastal vegetation is decreased by 23–27% due to estuarine CO₂e outgassing, resulting in a global median net sink of 391 or 444 TgCO₂e yr−1 using the 20- or 100-year global warming potentials, respectively. Globally, total coastal CH₄ and N₂O emissions decrease the coastal CO₂ sink by 9–20%. Southeast Asia, North America and Africa are critical regional hotspots of GHG sinks. Understanding these hotspots can guide our efforts to strengthen coastal CO₂ uptake while effectively reducing CH₄ and N₂O emissions.

link to the article
link to the press release

River ecosystem metabolism and carbon biogeochemistry in a changing world

Battin T.J., Lauerwald R., Bernhardt E.S., Bertuzzo E., Gener L.G., Hall Jr R.O., Hotchkiss E.R., Maavara T., Pavelsky T.M., Ran L., Raymond P., Rosentreter J.A. and Regnier P.

River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism
research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget.

link to the article

link to press release

Transfer efficiency of organic carbon in marine sediments

Bradley J.A., Hülse D., LaRowe D.E. and Arndt S.

Quantifying the organic carbon (OC) sink in marine sediments is crucial for assessing how the marine carbon cycle regulates Earth’s climate. However, burial efficiency (BE) – the commonly-used metric reporting the percentage of OC deposited on the seafloor that becomes buried (beyond an arbitrary and often unspecified reference depth) – is loosely defined, misleading, and inconsistent. Here, we use a global diagenetic model to highlight orders-of-magnitude differences in sediment ages at fixed sub-seafloor depths (and vice-versa), and vastly different BE’s depending on sediment depth or age horizons used to calculate BE. We propose using transfer efficiencies (Teff’s) for quantifying sediment OC burial: Teff is numerically equivalent to BE but requires precise specification of spatial or temporal references, and emphasizes that OC degradation continues beyond these horizons. Ultimately, quantifying OC burial with precise sediment-depth and sediment-age-resolved metrics will enable a more consistent and transferable assessment of OC fluxes through the Earth system.

link to the article

Some pictures of the brand new Belgica