Marked negative δ
13C excursions in Ediacaran-age carbonate sediments have been identified in several sections globally, but are not recognized in all sections of similar age. The presence of δ
13C
...carb values as low as −12‰ has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today. The δ
13C
carb anomalies are strongly correlated with δ
18O
carb values but are not represented in δ
13C
org records. While no primary depositional processes have been identified that can produce the correlated δ
18O–δ
13C arrays, simulations show that fluid–rock interaction with high-
pCO
2 fluids is capable of producing such arrays at geologically reasonable
pCO
2 and water–rock ratios. Variations in the Mg/Ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs. calcite and anhydrite in the resulting mineral assemblage. Incorporation of an initially aragonitic mineralogy demonstrates that high Sr, low Mn/Sr and modest alteration of
87Sr/
86Sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the δ
13C anomalies, and do not necessarily imply preservation of primary values. The profound Ediacaran negative δ
13C anomalies can be adequately explained by well-understood diagenetic processes, conflated with the difficulty of correlating Precambrian sections independently of chemostratigraphy. They are not a record of primary seawater variations and need not have independent stratigraphic significance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Evidence for low pO2 and a ferruginous ocean characterize the mid‐Proterozoic (1.8–0.8 Ga). Considerations of redox sources and sinks imply that generation of O2 via organic carbon (Corg) burial must ...be low to maintain a low pO2 atmosphere for geologically long intervals, yet low oxygen should result in increased Corg preservation. Marine export production must therefore be low to limit Corg burial and O2 generation. Formation of ferrous phosphate can buffer deepwater phosphate (Pi) to levels an order of magnitude or more below those in the modern ocean, limiting export production. Low deepwater Pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid‐Proterozoic, and low pO2 limits formation of sedimentary iron deposits (BIF). We propose that low pO2 was maintained by P limitation resulting from ferrous phosphate buffering. The near‐absence of BIF and phosphorite deposition is direct and indirect consequences of the low pO2, respectively.
Key Points
Ferrous phosphate buffering can limit O2 generation during the mid‐Proterozoic
Low export production is necessary to maintain low pO2 over long time scales
Low pO2 can limit the formation of BIF during the mid‐Proterozoic
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The balance between photosynthetic organic carbon production and respiration controls atmospheric composition and climate
. The majority of organic carbon is respired back to carbon dioxide in the ...biosphere, but a small fraction escapes remineralization and is preserved over geological timescales
. By removing reduced carbon from Earth's surface, this sequestration process promotes atmospheric oxygen accumulation
and carbon dioxide removal
. Two major mechanisms have been proposed to explain organic carbon preservation: selective preservation of biochemically unreactive compounds
and protection resulting from interactions with a mineral matrix
. Although both mechanisms can operate across a range of environments and timescales, their global relative importance on 1,000-year to 100,000-year timescales remains uncertain
. Here we present a global dataset of the distributions of organic carbon activation energy and corresponding radiocarbon ages in soils, sediments and dissolved organic carbon. We find that activation energy distributions broaden over time in all mineral-containing samples. This result requires increasing bond-strength diversity, consistent with the formation of organo-mineral bonds
but inconsistent with selective preservation. Radiocarbon ages further reveal that high-energy, mineral-bound organic carbon persists for millennia relative to low-energy, unbound organic carbon. Our results provide globally coherent evidence for the proposed
importance of mineral protection in promoting organic carbon preservation. We suggest that similar studies of bond-strength diversity in ancient sediments may reveal how and why organic carbon preservation-and thus atmospheric composition and climate-has varied over geological time.
We investigated the products and rates of chemical weathering on the Hawaiian Islands, sampling streams on Kaua’i and both streams and groundwater wells on the island of Hawai’i. Dissolved silica was ...used to investigate the flowpaths of water drained into streams. We found that flowpaths exert a major control on the observed chemical weathering rates. A strong link exists between the degree of landscape dissection and flowpaths of water through the landscape, with streams in undissected landscapes receiving water mainly from surface runoff and streams in highly dissected landscapes receiving a considerable fraction of their water from groundwater (springs and/or seepage). Total alkalinity in Hawaiian streams and groundwater is produced exclusively by silicate chemical weathering. We find that fluxes of total alkalinity (often called “CO2 consumption rate” in the geochemical literature), from the islands are lower than those observed in basaltic regions elsewhere. Groundwater is, overall, the major transport vector for products of chemical weathering from the Hawaiian Islands. On the youngest and largest island, submarine groundwater discharge (SGD) transports more than an order of magnitude more solutes to the ocean than surface water and on the youngest part of the youngest island, SGD is the only link between the terrestrial weathering system and the ocean. These results suggest that groundwater, and particularly SGD, needs to be included in geochemical weathering budgets of volcanic islands.
► Rates of chemical weathering were investigated on Kaua’i and the island of Hawai’i. ► Groundwater is the major export vector of chemical weathering products to the ocean. ► On Hawai’i, groundwater exports >10 times more solutes than surface water. ► Flowpaths of water determine the observed chemical weathering rates. ► Strong links exist between degree of landscape dissection and water flowpaths.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Critical zone science seeks to develop mechanistic theories that describe critical zone structure, function, and long‐term evolution. One postulate is that hydrogeochemical controls on critical zone ...evolution can be inferred from solute discharges measured down‐gradient of reactive flow paths. These flow paths have variable lengths, interfacial compositions, and residence times, and their mixing is reflected in concentration‐discharge (C‐Q) relations. Motivation for this special section originates from a U.S. Critical Zone Observatories workshop that was held at the University of New Hampshire, 20–22 July 2015. The workshop focused on resolving mechanistic CZ controls over surface water chemical dynamics across the full range of lithogenic (e.g., nonhydrolyzing and hydrolyzing cations and oxyanions) and bioactive solutes (e.g., organic and inorganic forms of C, N, P, and S), including dissolved and colloidal species that may cooccur for a given element. Papers submitted to this special section on “concentration‐discharge relations in the critical zone” include those from authors who attended the workshop, as well as others who responded to the open solicitation. Submissions were invited that utilized information pertaining to internal, integrated catchment function (relations between hydrology, biogeochemistry, and landscape structure) to help illuminate controls on observed C‐Q relations.
Plain Language Summary
This article introduces the special section of Water Resources Research entitled “Concentration‐Discharge Relations in the Critical Zone”.
Key Points
Interdisciplinary critical zone science can provide transformative insights into controls on concentration‐discharge relations observed in streams and rivers
Critical zone scientists and observatories are making hydrologic and hydrochemical measurements using common approaches that enable comparison across sites
This special section contains a set of papers wherein insights into critical zone structure and function are applied to interpret catchment concentration‐discharge relations
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We have investigated how biota contributes to rapid chemical weathering of Hawaiian basalts using a reactive transport model and chemical data from a soil chronosequence. These Hawaiian soils have ...developed under a tropical forest with rainfall >200 cm/yr and exhibit extensive weathering on timescales of 104 years. We developed a series of multicomponent reactive transport models to examine the role of soil respiration and low molecular weight organic acids in generating these intense weathering patterns. The base model starts with a 1-m basaltic porous media reacting with a fluid of rainwater composition in equilibrium with atmospheric CO2. Subsequent simulations incorporate soil respiration modeled as a constant flux of CO2 at 10× atmospheric and continuous input of organic ligands – oxalate and citrate – at 10−4 molar. After 20 kyr of weathering, the base model shows limited elemental losses, high soil pH and is overall CO2(acid)-limited. Soil respiration lowers soil pH to circumneutral values, leaches all Mg and Ca from the basalt and allows precipitation of Fe(III)-oxyhydroxides, while Al stays immobile as secondary clays accumulate. After adding organic ligands, soil pH is reduced to values similar to the Hawaiian soils and Si, Al and Fe are exported from the system by dissolution of secondary phases, resulting in mass depletion patterns similar to the ones observed in Hawai’i. Dissolution of secondary minerals is generated by low pH and relatively low free activities of Al3+ and Fe3+ when organic ligands are added. These results suggest that organic acids in basalt weathering in tropical environments can sustain far-from-equilibrium conditions that drive fast elemental losses and that biologic activity contributes to weathering processes both by generating high soil PCO2 and organic acids.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The homeostatic balance of Earth's long-term carbon cycle and the equable state of Earth's climate are maintained by negative feedbacks between the levels of atmospheric CO2 and the chemical ...weathering rate of silicate rocks. Though clearly demonstrated by well-controlled laboratory dissolution experiments, the temperature dependence of silicate weathering rates, hypothesized to play a central role in these weathering feedbacks, has been difficult to quantify clearly in natural settings at landscape scale. By compiling data from basaltic catchments worldwide and considering only inactive volcanic fields (IVFs), here we show that the rate of CO2 consumption associated with the weathering of basaltic rocks is strongly correlated with mean annual temperature (MAT) as predicted by chemical kinetics. Relations between temperature and CO2 consumption rate for active volcanic fields (AVFs) are complicated by other factors such as eruption age, hydrothermal activity, and hydrological complexities. On the basis of this updated data compilation we are not able to distinguish whether or not there is a significant runoff control on basalt weathering rates. Nonetheless, the simple temperature control as observed in this global dataset implies that basalt weathering could be an effective mechanism for Earth to modulate long-term carbon cycle perturbations.
•A compilation of CO2 consumption rates by basalt weathering is presented.•Basalt weathering rates show strong dependence on temperature.•Basalt weathering provides negative feedback in Earth's long-term carbon cycle.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Reconstructions of the carbonate compensation depth (CCD) in the past have been used to inform hypotheses about the nature of weathering, tectonics, climate change, and the major ion content of the ...world's oceans over the Cenozoic. These reconstructions are sensitive to uncertainties in the input data, in particular, the paleodepth estimates of sediment cores. Here we propose that a significant, previously unconsidered contributor to uncertainties in paleodepth estimates is from dynamic topography produced by radial stresses exerted on the Earth's surface by the convecting mantle; these stresses can warp the ocean floor by hundreds of meters over broad regions and also vary significantly over millions of years. We present new reconstructions of the equatorial Pacific and Indian Ocean CCDs over the last 30 and 23 Myr, respectively, which demonstrate an overall deepening trend since the Miocene, and illustrate the possible effect of long‐term changes in dynamic topography on these reconstructions.
Key Points
Reconstructions of the carbonate compensation depth require that changes in dynamic topography be considered
The carbonate compensation depths of both the Indian and Pacific Oceans reflect an overall deepening trend since the Miocene
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Silicon has a crucial role in many biogeochemical processesfor example, as a nutrient for marine and terrestrial biota, in buffering soil acidification and in the regulation of atmospheric carbon ...dioxide. Traditionally, silica fluxes to soil solutions and stream waters are thought to be controlled by the weathering and subsequent dissolution of silicate minerals. Rates of mineral dissolution can be enhanced by biological processes. But plants also take up considerable quantities of silica from soil solution, which is recycled into the soil from falling litter in a separate soil-plant silica cycle that can be significant in comparison with weathering input and hydrologic output. Here we analyse soil water in basaltic soils across the Hawaiian islands to assess the relative contributions of weathering and biogenic silica cycling by using the distinct signatures of the two processes in germanium/silicon ratios. Our data imply that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, whereas direct mineral-water reactions account for a smaller fraction of the stream silica flux. We expect that other systems exhibiting strong Si depletion of the mineral soils and/or high Si uptake rates by biomass will also have strong biological control on silica cycling and export.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The carbonate compensation depth (CCD), δ13C of marine carbonate, atmospheric pCO2 and major ion composition of seawater provide constraints on how geological carbon cycle processes evolved over the ...Neogene. I use simple models and the LOSCAR ocean carbon system model to assess what changes in carbon fluxes to the ocean are necessary to explain observations since the early Miocene. The calculations consider estimates of early Miocene seawater temperatures and ion composition and a range of possible pCO2. Changes in shelf-basin partition could explain up to ≈45% the observed CCD deepening. Increased carbonate flux (likely range 28±12%) to the oceans is necessary to explain the rest. Despite changes in pCO2 from early Miocene values of 450–900 ppm to a pre-anthropogenic value of 280 ppm, the size of the total ocean-atmosphere carbon reservoir shows only moderate or no net change, implying that weathering and/or organic carbon burial result in little net CO2 consumption. Decreasing Ca++ and increasing deepwater carbonate saturation over the Neogene require a large increase in deepwater CO=3 and leads to decreasing DIC/TALK which is the main driver for falling pCO2. The primary driver of pCO2 reduction is redistribution of CO2 from the atmosphere to the oceans, not net removal of CO2 from excess silicate weathering or organic carbon burial. The main impact of tectonic perturbation of the carbon cycle during the Neogene is to enhance carbonate weathering while only weakly affecting the net balance of degassing vs. silicate weathering or kerogen oxidation vs. organic carbon burial.
•The carbonate weathering flux increased by ca. 30% from the early Miocene to the present.•Changes in the relative size of the ocean-atmosphere C reservoir were much less than for pCO2.•Lower ocean Ca++ and increasing Ωcalcite redistribute CO2 from the atmosphere into the oceans.•Changes in ocean chemistry can lower pCO2 without silicate or organic carbon imbalances.•Net tectonic influence on the Neogene carbon cycle is largely a carbonate weathering event.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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