DOI: 10.34894/kgrysh
title: Supplementary Data to PhD thesis 'Earth's Weathering Continuum' (Gerrit Trapp-Müller)
title: Link between weathering, terrigenous marine sediment, and global biogeochemistry

description: The chemical weathering of rocks, e.g., during soil formation, leads to a global scale redistribution of the chemical elements. Weathering on land typically releases nutrients otherwise trapped in rocks and transforms atmospheric carbon dioxide (CO2 dissolved in rain) to dissolved bicarbonate (HCO3-) at rates that appear to accelerate in a warming climate. Because CO2 has a warming effect on climate (‘greenhouse’ effect), weathering is thought to exert carbon-cycle feedback that stabilizes life-supporting Earth surface conditions on long, geological timescales. The relevant timescales depend on the material weathered: Limestone (carbonate minerals, mostly (Ca,Mg)CO3) weathers rapidly but its chemical effects are largely reversed by biological calcification (CaCO3 biomineralization) in the ocean after ~ 50,000 – 100,000 years. However, reorganisation of carbonate sources and sinks within the ocean can drive long-term effects. The bicarbonate released during weathering of silicate rocks (various Si-based minerals) is only partly offset by calcification, hence silicate weathering net sequesters atmospheric CO2, and it also adds salts and nutrients to the oceans. However, despite this relevance to seawater chemistry and Earth system regulation, the role of the weathering feedback and its relationship to biology, tectonics and climate remain enigmatic, impeding our understanding of past and future Earth system evolution. Traditionally, the weathering of continental and oceanic crusts is considered in biogeochemical cycles and Earth system models, but many biogeochemical budgets cannot yet consentaneously be balanced. Moreover, variable weathering responses to carbon cycle perturbations have been demonstrated and remain to be consistently explained. In contrast, the weathering of eroded soils and rocks in marine sediments has largely been ignored, although the widespread occurrence and its relevance to Earth system dynamics have been demonstrated quite early (Chapter 1). We now know that marine sediment weathering quantitatively rivals crustal and land weathering, but the magnitude and even direction of the corresponding chemical fluxes (incl. CO2) vary extensively between different settings and even with depth and time at individual locations. In this PhD thesis, I systematically investigate the role of marine sediment weathering, i.e., chemical reactions between rock and soil-derived minerals, biological remains and seawater, in the global cycles of the element and in the regulation of planetary conditions.
Because most of the sediment on the ocean margins originates from land and is transported by rivers, I assembled a comprehensible reference database of river sediment composition (GloRiSe, Chapter 2). This allowed mapping how much of which particle type is discharged by which rivers and what governs the relative abundances of different particles. By comparing the database to various maps of the Earth’s surface and applying statistical techniques, a seamless, global map of eroded limestone fragments (‘detrital carbonates’) discharge by rivers to the ocean was derived (Chapter 3). These data point to a quantitative significance of riverine detrital carbonates for the global biogeochemical cycles of carbon, calcium, alkalinity, and strontium. This detrital carbonate flux was not previously considered and narrows gaps that persisted in corresponding biogeochemical cycles for decades. Variation with hydroclimate, land surface properties and tectonically controlled distributions of limestones are the main factors governing the detrital carbonate fluxes. Challenges remain in the assessment of non-riverine sources of detrital carbonate, particularly coastal erosion, and in quantifying the fate of these minerals in the ocean, where they may either dissolve into seawater or act as ‘seeds’ for inorganic carbonate precipitation from seawater. 
The remainder of the sediment reaching the ocean is largely made of eroded silicate rock fragments and soil materials, including clays, organic matter, and metal (oxy-)hydroxides (rust). Our data support the view that weathering intensities on land are highest in tropical rivers (latitudinal pattern), broadly consistent with the traditional view of the weathering feedback. Moreover, our database revealed a prominent role of bedrock types and that the degree of weathering of river sediments often increases downstream, emphasizing the roles of tectonics, topography, and hydrology in determining material export fluxes. These data demonstrate significant, climate-dependent pre-processing of these materials before they reach the coasts, with implications for their fate in the ocean. Deltas are the main entry point for sediment in the ocean and represent dynamic and efficient chemical reactors. Using a chemical reaction model, we found systematic weathering patterns in such deltaic sediments (Chapter 5). Acidification, CO2 mobilization, and element consumption (K, Mg, Fe, Si) through so-called ‘reverse’ weathering is promoted in seasonally reworked, low-latitude deltaic muds, where iron-rich ‘green clays’ form rapidly from reactions of lateritic soil materials and plankton remains (including SiO2 biominerals). In contrast, element release (Na, Ca, Mg, Fe, Si), carbon storage and/or alkalinization by sediment weathering are most likely where pristine rock fragments and ashes weather in organic-rich and methane-generating sediments. These reactions would be promoted on continental slopes with nearby mountains and volcanoes, and at high latitudes, where physical erosion readily provides relatively pristine and weatherable rock fragments. Notably, silicate weathering in marine sediments is intimately tied to various other biogeochemical processes (relative to organics, phosphates, oxides, sulfides, carbonates), with potentially far-reaching consequences for nutrient and carbon cycling on local to global scales. 
I conclude that marine sediment weathering is most accurately thought of as a continuum of reaction balances, moderated by sediment sources (chemical reactant mixes) and by the depositional environment (Chapter 4-6). The diverse range of marine sedimentary environments produces various weathering reaction balances and fluxes that need to be considered in concert to derive global chemical fluxes (Chapter 6). Prominent ‘endmembers’ with fundamentally contrasting weathering dynamics and reaction balances and that are quantitatively significant on the global scale are (I) beaches, rocky coasts, and permeable sediment, (II) muddy river deltas, (III) continental slopes (active vs. passive margins), (IV) deep-sea clays, (V) siliceous oozes, (VI) volcanic and hydrothermal environments, and (VII) carbonate oozes. These results demonstrate how Earth’s weathering feedback may be governed by a continuum of downstream connected weathering reactions extending from the highest mountains to the deepest hadal trenches, moderated by transport and local boundary conditions (Chapter 7). Moreover, we find that deltas and beaches are weathering hotspots of the modern ocean. Clearly, the Earth’s weathering continuum is shaped by physical, chemical, and biological processes, by climatic and oceanic forces and those of the Earth’s interior. An integral understanding of these weathering continuum dynamics will progress our understanding of the behaviour of and role of humans in the Earth system, and aid targeted geo-engineering for the benefit of nature and society.