scholarly journals Reconciling atmospheric CO2, weathering, and calcite compensation depth across the Cenozoic

2021 ◽  
Vol 7 (4) ◽  
pp. eabd4876
Author(s):  
Nemanja Komar ◽  
Richard E. Zeebe
Keyword(s):  
Carbon Cycle ◽  
Critical Role ◽  
Carbonate Weathering ◽  
Weathering Rates ◽  
Marine Organic Matter ◽  
Novel Approach ◽  
Compensation Depth ◽  
Long Term Trends ◽  
Cenozoic Era

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 pCO2, 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.

scholarly journals Deepening roots can enhance carbonate weathering

2020 ◽  
Author(s):  
Hang Wen ◽  
Pamela L. Sullivan ◽  
Gwendolyn L. Macpherson ◽  
Sharon A. Billings ◽  
Li Li
Keyword(s):  
Carbon Cycle ◽  
Reactive Transport ◽  
Numerical Experiments ◽  
Chemical Weathering ◽  
Dissolved Inorganic Carbon ◽  
Base Case ◽  
Reaction Products ◽  
Soil Co2 ◽  
Carbonate Weathering ◽  
Weathering Rates

Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century time scale. Plant roots have been known to accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots potentially enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and Dissolved Inorganic Carbon (DIC) are regulated by soil pCO2 driven by the seasonal fluctuation of soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2, which has been shown to apply in multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating the amount of water fluxes that flush through the carbonate zone and export reaction products at dissolution equilibrium. Numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data from wood- and grasslands suggest relatively similar soil CO2 distribution over depth, and only led to 1 % to 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by 17 % to 207 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/yr. Numerical experiments also indicated that weathering fronts in woodlands propagated > 2 times deeper compared to grasslands after 300 years at the infiltration rate of 0.37 m/yr. These differences in weathering fronts are ultimately caused by the contact time of CO2-charged water with carbonate rocks. We recognize that modeling results are subject to limitations in representing processes and parameters, but we propose that the data and numerical experiments allude to the hypothesis that (1) deepening roots can enhance carbonate weathering; (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for co-located characterizations of roots, subsurface structure, soil CO2 levels, and their linkage to water and water chemistry. These measurements will be essential to improve models and illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

scholarly journals Long-term summer temperature variations in the Pyrenees from detrended stable carbon isotopes

2015 ◽  
Vol 42 (1) ◽  
Author(s):  
Jan Esper ◽  
Oliver Konter ◽  
Paul J. Krusic ◽  
Matthias Saurer ◽  
Steffen Holzkämper ◽  
...  
Keyword(s):  
Carbon Isotopes ◽  
Tree Ring ◽  
Stable Carbon Isotopes ◽  
Summer Temperature ◽  
Stable Carbon ◽  
Novel Approach ◽  
Decadal Variations ◽  
Temperature Signal ◽  
Long Term Trends

Abstract Substantial effort has recently been put into the development of climate reconstructions from tree-ring stable carbon isotopes, though the interpretation of long-term trends retained in such timeseries remains challenging. Here we use detrended δ13C measurements in Pinus uncinata tree-rings, from the Spanish Pyrenees, to reconstruct decadal variations in summer temperature back to the 13th century. The June-August temperature signal of this reconstruction is attributed using decadally as well as annually resolved, 20th century δ13C data. Results indicate that late 20th century warming has not been unique within the context of the past 750 years. Our reconstruction contains greater am-plitude than previous reconstructions derived from traditional tree-ring density data, and describes particularly cool conditions during the late 19th century. Some of these differences, including early warm periods in the 14th and 17th centuries, have been retained via δ13C timeseries detrending - a novel approach in tree-ring stable isotope chronology development. The overall reduced variance in earlier studies points to an underestimation of pre-instrumental summer temperature variability de-rived from traditional tree-ring parameters.

Processes of the Long-Term Carbon Cycle: Organic Matter and Carbonate Burial and Weathering

2004 ◽  
Author(s):  
Robert A. Berner
Keyword(s):  
Organic Matter ◽  
Carbon Cycle ◽  
Atmospheric Oxygen ◽  
Marine Phytoplankton ◽  
Atmospheric Co2 ◽  
Carbonate Weathering ◽  
Major Question ◽  
Water Composition ◽  
Subsequent Exposure

The organic subcycle of the long-term carbon cycle, where organic matter burial and weathering are involved, constitutes the major control on the evolution of atmospheric oxygen. It is also important as a secondary factor affecting atmospheric CO2. Thus, it is important to better understand the processes whereby organic matter is buried in sediments and oxidized upon subsequent exposure to weathering during uplift onto the continents. This is especially true of the Paleozoic rise of land plants, which had a large effect on atmospheric CO2 because of increased global organic burial due to the addition of plant debris to sediments. The burial of organic matter in marine sediments is impacted strongly by the availability of the nutrient elements, phosphorus and nitrogen, so a complete discussion of the cycling of organic carbon should involve some discussion of the cycles of these elements. Carbonate burial is the ultimate sink for CO2 derived from the atmosphere via the weathering of Ca and Mg silicates. The location of this burial, shallow water shelves versus the deep sea floor, is important because it affects the probability that the carbonate will be eventually thermally recycled and the carbon returned to the atmosphere. Carbonate weathering is the dominant process affecting river water composition and is a key component of the cycling of carbon. Its importance to the long-term carbon cycle is that, in order to calculate the removal of CO2 from the atmosphere via Ca and Mg silicate weathering, it is necessary to correct total carbonate burial for that derived from carbonate weathering. At present, sedimentary organic matter burial occurs in swamps, lakes, reservoirs, estuaries, and in the open marine environment. The ultimate sources of the organics are land vegetation and marine phytoplankton. Also, soil organic matter, which is intimately associated with clay minerals, is eroded and transported to the sea by rivers (Hedges et al., 1994). A major question is how much of the total global burial is of marine or nonmarine origin. Recent work has shown that organic burial on land is much higher than previously recognized, especially as a result of human activities (Dean and Gorham, 1998; Stallard, 1998).

Weathering of soil minerals by angiosperm and gymnosperm trees

2008 ◽  
Vol 72 (1) ◽  
pp. 11-14 ◽  
Author(s):  
M. Y. Andrews ◽  
J. J. Ague ◽  
R. A. Berner
Keyword(s):  
Carbon Cycle ◽  
Vascular Plants ◽  
Mineral Weathering ◽  
Large Role ◽  
Geological Time ◽  
Weathering Rates ◽  
Element Fluxes ◽  
Ecological Dominance ◽  
Field Setting

AbstractWeathering of terrestrial Ca- and Mg-bearing silicate minerals is an important control on atmospheric CO2 on geological time scales. It has been determined that vascular plants can accelerate mineral weathering as compared to non-vascular plants or non-vegetated surfaces. This indicates that the evolution of vascular plants, particularly the deep-rooted trees, may play a large role in the long-term carbon cycle and its regulation of the atmosphere. The weathering impact of the separate evolutionary appearances of the gymnosperms in the Palaeozoic and the angiosperms in the Mesozoic, and the shifting ecological dominance from the former to the latter, is currently poorly understood. This study aims to contribute to our understanding of the quantitative weathering rates of the angiosperms and gymnosperms by examining plant-mineral interactions of the two tree types in a temperate field setting underlain by granodiorite. Results include determinations of soil element fluxes and etching of minerals. The observed root-mineral interactions resulted in only slightly more weathering of Ca-bearing minerals by the angiosperms. However, we observed significantly more weathering of the Mg-bearing minerals by the gymnosperms. These results suggest that increasing dominance of the angiosperms in forests in the Mesozoic may have had a small or neutral impact on accelerating overall mineral weathering and regulating CO2, but that this impact may be lithology-dependent.

The Role of Lithology in Silicate Weathering and CO2 Regulation on Rocky Exoplanets

2020 ◽  
Author(s):  
Kaustubh Hakim ◽  
Dan J. Bower ◽  
Meng Tian ◽  
Russell Deitrick ◽  
Pierre Auclair-Desrotour ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Carbon Cycle ◽  
Theoretical Method ◽  
Silicate Weathering ◽  
Weathering Rates ◽  
Rock Water Interaction ◽  
Tectonically Active ◽  
Strong Control

<p>In the decade of JWST, ELT, TMT, PLATO, ARIEL and other specialized telescopes, observations of carbon dioxide in terrestrial exoplanet atmospheres are possible. The amount of carbon dioxide in the atmosphere of a tectonically active planet such as Earth is regulated by the carbonate-silicate cycle (long-term carbon cycle). Silicate weathering provides essential negative feedback to maintain temperate climates on Earth over billions of years. In this study, we model the chemistry of rock-water interaction for different silicate rocks and minerals applicable to both continental and seafloor weathering. We find that weathering rates depend mainly on the partial pressure of carbon dioxide, surface temperature and lithology, and other factors are secondary. This approach allows possessing a theoretical method to determine both continental and seafloor weathering rates on temperate exoplanets that depend little on present-day Earth calibrations. Our study gives a strong control over the connection between atmospheric observables and the carbon cycle. The ultimate goal is to provide an abiotic library of geological false positives of biosignatures.</p>

scholarly journals Deepening roots can enhance carbonate weathering by amplifying CO<sub>2</sub>-rich recharge

2021 ◽  
Vol 18 (1) ◽  
pp. 55-75
Author(s):  
Hang Wen ◽  
Pamela L. Sullivan ◽  
Gwendolyn L. Macpherson ◽  
Sharon A. Billings ◽  
Li Li
Keyword(s):  
Carbon Cycle ◽  
Reactive Transport ◽  
Numerical Experiments ◽  
Chemical Weathering ◽  
Dissolved Inorganic Carbon ◽  
Base Case ◽  
Soil Co2 ◽  
Carbonate Weathering ◽  
Deep Subsurface ◽  
Weathering Rates

Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century timescale. Plant roots accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports vertical downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are regulated by soil pCO2 driven by the seasonal soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating recharge (or vertical water fluxes) into the deeper carbonate zone and export reaction products at dissolution equilibrium. The numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data suggest relatively similar soil CO2 distribution over depth, which in woodlands and grasslands leads only to 1 % to ∼ 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to 200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots at all, underscoring the essential role of roots in general. Numerical experiments also indicate that weathering fronts in woodlands propagated > 2 times deeper compared to grasslands after 300 years at an infiltration rate of 0.37 m/a. These differences in weathering fronts are ultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promoting recharge and CO2–carbonate contact in the deep subsurface and (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for colocated characterizations of roots, subsurface structure, and soil CO2 levels, as well as their linkage to water and water chemistry. These measurements will be essential to illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

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