| Summary: | Marine sediments are key components of the Earth system. They represent one of the largest active pools of organic matter and provide long-term sink for CO2 and CH4. Thus, the biogeochemical processes that take place in those sediments are crucial for the global carbon cycle and Earth climate. Organic matter reactivity plays a key role on organic matter degradation and burial, and thus on biogeochemical processes. Despite of its importance, the environmental controls on reactivity are poorly understood on global-scale. The lack of comprehensive, multidisciplinary approaches investigating the controls on reactivity limits our ability to further understand biogeochemical processes on global and different time-scales. Here, this problem is tackled in coupled quantitative investigation of the relationships between organic matter reactivity and sources. A large-scale compilation of contrasting depositional environments is systematically probed through consistent model parametrization for quantifying reactivity and laboratory protocols for determining lipid biomarker compositions. On global-scale, organic matter reactivity exhibits weak correlations with single characteristics of depositional environments, albeit on regional-scales a few patterns emerge. Similarly, the organic matter compositions show a broad global pattern with a terrestrial-to-marine shift in sources with increase of water depth, although different lipid proxies behaved differently across this spectrum. The environmental controls on reactivity are complex, and lipid biomarker compositions only offer robust information when considered in the broad environmental context of each depositional regime. With this holistic view, the controls on organic matter reactivity are better understood on global-scale. Those findings challenge the classical view of simple mechanisms and single characteristics control on reactivity. Yet, those findings have direct impact on model parametrization since they help to identify reasonable intervals of reactivity parameters in different depositional regimes. Ultimately, this results in better predictive capability and helps to better constrain perturbations on the past, present, and future carbon cycle and climate.
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