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1. Volcanic Arc Weathering Rates in the Humid Tropics Controlled by the Interplay Between Physical Erosion and Precipitation

   https://doi.org/10.1029/2023AV001066  

   Moore, Angus_K; Méndez_Méndez, Kimberly; Hughes, K_Stephen; Granger, Darryl_E (March 2024, AGU Advances)

   Abstract Volcanic arcs are chemical weathering hotspots that may contribute disproportionately to global CO₂consumption through silicate weathering. Accurately modeling the impact of volcanic‐arc landscapes on the Earth's long‐term carbon cycle requires understanding how climate and physical erosion control weathering fluxes from arc landscapes. We evaluate these controls by examining the covariation of stream solutes, sediment geochemistry, and long‐term physical erosion fluxes inferred from cosmogenic³⁶Cl in magnetite in volcanic watersheds in Puerto Rico that span a ca. 15‐fold gradient in specific discharge. Analysis of this data using power‐law relationships demonstrates that CO₂consumption from arc‐rock weathering in the humid tropics is more strongly limited by physical erosion and the supply of primary minerals to the weathering zone than by temperature or the flux of fresh, chemically reactive waters through the critical zone. However, a positive correlation between long‐term physical erosion fluxes and specific discharge is also observed. This indicates that fresh mineral supply in arc environments may ultimately depend on precipitation rates, which may maintain a coupling between arc‐rock weathering fluxes and climate under principally supply limited weathering conditions. 

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2. K isotopes as a tracer for continental weathering and geological K cycling

   https://doi.org/10.1073/pnas.1811282116  

   Li, Shilei; Li, Weiqiang; Beard, Brian L.; Raymo, Maureen E.; Wang, Xiaomin; Chen, Yang; Chen, Jun (April 2019, Proceedings of the National Academy of Sciences)

   The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ⁴¹K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ⁴¹K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ⁴¹K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ⁴¹K record of paleo-seawater to infer continental weathering intensity through Earth’s history. 

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3. Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends

   https://doi.org/10.1029/2019PA003601  

   Greene, S. E.; Ridgwell, A.; Kirtland Turner, S.; Schmidt, D. N.; Pälike, H.; Thomas, E.; Greene, L. K.; Hoogakker, B. A. A. (June 2019, Paleoceanography and Paleoclimatology)

   Abstract Our understanding of the long‐term evolution of the Earth system is based on the assumption that terrestrial weathering rates should respond to, and hence help regulate, atmospheric CO₂and climate. Increased terrestrial weathering requires increased carbonate accumulation in marine sediments, which in turn is expected to result in a long‐term deepening of the carbonate compensation depth (CCD). Here, we critically assess this long‐term relationship between climate and carbon cycling. We generate a record of marine deep‐sea carbonate abundance from selected late Paleocene through early Eocene time slices to reconstruct the position of the CCD. Although our data set allows for a modest CCD deepening, we find no statistically significant change in the CCD despite >3 °C global warming, highlighting the need for additional deep‐sea constraints on carbonate accumulation. Using an Earth system model, we show that the impact of warming and increased weathering on the CCD can be obscured by the opposing influences of ocean circulation patterns and sedimentary respiration of organic matter. From our data synthesis and modeling, we suggest that observations of warming, declining δ¹³C and a relatively stable CCD can be broadly reproduced by mid‐Paleogene increases in volcanic CO₂outgassing and weathering. However, remaining data‐model discrepancies hint at missing processes in our model, most likely involving the preservation and burial of organic carbon. Our finding of a decoupling between the CCD and global marine carbonate burial rates means that considerable care is needed in attempting to use the CCD to directly gauge global carbonate burial rates and hence weathering rates. 

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4. Reconciling atmospheric CO ₂ , weathering, and calcite compensation depth across the Cenozoic

   https://doi.org/10.1126/sciadv.abd4876  

   Komar, Nemanja; Zeebe, Richard E. (January 2021, Science Advances)

   null (Ed.)

   The Cenozoic era (66 to 0 million years) is marked by long-term aberrations in carbon cycling and large climatic shifts, some of which challenge the current understanding of carbon cycle dynamics. Here, we investigate possible mechanisms responsible for the observed long-term trends by using a novel approach that features a full-fledged ocean carbonate chemistry model. Using a compilation of pCO 2 , pH, and calcite compensation depth (CCD) observational evidence and a suite of simulations, we reconcile long-term Cenozoic climate and CCD trends. We show that the CCD response was decoupled from changes in silicate and carbonate weathering rates, challenging the continental uplift hypothesis. The two dominant mechanisms for decoupling are shelf-basin carbonate burial fractionation combined with proliferation of pelagic calcifiers. The temperature effect on remineralization rates of marine organic matter also plays a critical role in controlling the carbon cycle dynamics, especially during the warmer periods of the Cenozoic. 

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5. The effect of the Pliocene temperature pattern on silicate weathering and Pliocene–Pleistocene cooling

   https://doi.org/10.5194/cp-19-1461-2023  

   Maffre, Pierre; Chiang, John_C H; Swanson-Hysell, Nicholas L (July 2023, Climate of the Past)

   Abstract. The warmer early Pliocene climate featured changes to global sea surface temperature (SST) patterns, namely a reduction in the Equator–pole gradient and the east–west SST gradient in the tropical Pacific, the so-called “permanent El Niño”. Here we investigate the consequences of the SST changes to silicate weathering and thus to atmospheric CO2 on geological timescales. Different SST patterns than today imply regional modifications of the hydrological cycle that directly affect continental silicate weathering in particular over tropical “hotspots” of weathering, such as the Maritime Continent, thus leading to a “weatherability pattern effect”. We explore the impact of Pliocene-like SST changes on weathering using climate model and silicate weathering model simulations, and we deduce CO2 and temperature at carbon cycle equilibrium between solid Earth degassing and silicate weathering. In general, we find large regional increases and decreases in weathering fluxes, and the net effect depends on the extent to which they cancel. Permanent El Niño conditions lead to a small amplification of warming relative to the present day by 0.4 ∘C, suggesting that the demise of the permanent El Niño could have had a small amplifying effect on cooling from the early Pliocene into the Pleistocene. For the reducedEquator–pole gradient, the weathering increases and decreases largely cancel, leading to no detectable difference in global temperature at carbon cycle equilibrium. A robust SST reconstruction of the Pliocene is needed for a quantitative evaluation of the weatherability pattern effect. 

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