Lee R. Kump

Interpreting carbon-isotope excursions: carbonates and organic matter

Abstract Variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon. Paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time, but changes in atmospheric pCO2 as well. Interpretations of these types of data are typically rather qualitative; a quantitative basis is required to develop a better understanding of changes in the carbon cycle. For this purpose, we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations, each lasting 500 ky, i.e., much longer than the residence times of carbon and phosphorus in the ocean–atmosphere system. In addition to standard considerations of carbon mass and isotopic fluxes to the ocean–atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate–carbon to sediments, the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pCO2. The inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial. A series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon (ΔB). With constant ΔB a 50% increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions. However, the same simulation with ΔB responsive to pCO2 changes produces an initial parallel δ 13 C increase, but this is followed by an even greater 13 C -enrichment in organic carbon because pCO2 falls in response to increased organic carbon burial. The counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in δ 13 C org . In addition, we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50% increase in volcanic CO2 outgassing, a 50% increase in weathering rate (with coupled phosphate and riverine carbon flux responses), a 50% decrease in shale-associated organic carbon weathering, a 50% decrease in silicate weathering rate, and the possible effects of the rise in abundance of C4 plants in the late Miocene to Recent. We compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate- and organic-carbon records to illustrate how these records might be interpreted in light of the model response.

Lithologic and climatologic controls of river chemistry

Abstract The chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff. In all, 101 different rivers, each draining a primary lithology, were examined across the United States, Puerto Rico, and Iceland. To minimize seasonal effects, only rivers with at least two years of data were used. Basaltic catchments were examined in the greatest detail. In a survey of Hawaiian Island watersheds, the average river chemistries could be related to the distribution of soil associations within each catchment. An analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium (ion exchange) control on cation ratios. Among different lithologies, unique weathering rate relationships were developed with yields at typical present-day runoff rates (1–100 cm/y) increasing in the order sandstones, granites, basalts, shales, and carbonates. The bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation. Our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering; thus, a warm, wet climate, or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal.

Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia

Simple calculations show that if deep-water H2S concentrations increased beyond a critical threshold during oceanic anoxic intervals of Earth history, the chemocline separating sulfidic deep waters from oxygenated surface waters could have risen abruptly to the ocean surface (a chemocline upward excursion). Atmospheric photochemical modeling indicates that resulting fluxes of H 2S to the atmosphere (.2000 times the small modern flux from volcanoes) would likely have led to toxic levels of H 2S in the atmosphere. Moreover, the ozone shield would have been destroyed, and methane levels would have risen to .100 ppm. We thus propose (1) chemocline upward excursion as a kill mechanism during the end-Permian, Late Devonian, and Cenomanian‐Turonian extinctions, and (2) persistently high atmospheric H2S levels as a factor that impeded evolution of eukaryotic life on land during the Proterozoic.

The influence of temperature and pH on trace metal speciation in seawater

Abstract Using available data we have constructed complexation schemes which depict the influence of temperature and pH on metal speciation in seawater. Our calculations show that the extent of complexation of strongly hydrolyzed metals in seawater is strongly temperature and pH dependent. The extent of complexation of the twenty or more metals present in seawater predominantly as carbonate complexes is substantially influenced by pH and temperature, but to a much smaller degree than strongly hydrolyzed metals. For the small group of metals extensively complexed with chloride ions in seawater, temperature and pH appear to be variables of minor significance. We have identified the following concerns. (i) In general, previous experimental work does not provide well defined descriptions of metal hydrolysis under weakly alkaline conditions. (ii) Very few studies have been devoted to characterizing the temperature dependence of carbonate complexation equilibria. (iii) There are major uncertainties in the primary data upon which characterizations of Pd (II), Pt (II), Au (I), Ag (I) and Cu (I) chemistry are based.

The rise of atmospheric oxygen.

Clues from ancient rocks are helping to produce a coherent picture of how Earths atmosphere changed from one that was almost devoid of oxygen to one that is one-fifth oxygen.

A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician

New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO2, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO2 through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO2 decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO2 reached a level which overwhelmed the cooling effects of ice albedo, and the glaciation ended. The isotope results can be interpreted to indicate that atmospheric pCO2 rose during the glaciation, consistent with other proxy information, although alternative interpretations are possible. The large, positive carbonate isotope excursion observed in Late Ordovician rocks around the world is explained as the expected response to increased carbonate-platform weathering during glacioeustatic sea-level lowstand, rather than as a response to increased organic-carbon burial.

Geochemical evidence for widespread euxinia in the Later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.

Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago

The hypothesis that the establishment of a permanently oxygenated atmosphere at the Archaean–Proterozoic transition (∼2.5 billion years ago) occurred when oxygen-producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older. To sustain vanishingly low oxygen levels despite near-modern rates of oxygen production from ∼2.7–2.5 billion years ago thus requires that oxygen sinks must have been much larger than they are now. Here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the Archaean era—enhanced submarine volcanism—was abruptly and permanently diminished during the Archaean–Proterozoic transition. Observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the Proterozoic. Submarine volcanoes are more reducing than subaerial volcanoes, so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen.

Ocean stagnation and end-Permian anoxia

Ocean stagnation has been invoked to explain the widespread occurrence of organic-carbon–rich, laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end-Permian mass extinction. However, to a first approximation, stagnation would severely reduce the upwelling supply of nutrients to the photic zone, reducing productivity. Moreover, it is not obvious that ocean stagnation can be achieved. Numerical experiments performed with a three-dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the Late Permian ocean. We find that polar warming and tropical cooling of sea-surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization. Buildup of quantities of H2S and CO2 in the Late Permian ocean sufficient to directly cause a mass extinction, however, would have required large increases in the oceanic nutrient inventory.

Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison

Possible sources of carbon that may have caused global warming at the Paleocene-Eocene boundary are constrained using an intermediate complexity Earth-system model confi gured with early Eocene paleogeography. We fi that 6800 Pg C (δ 13 C of -22‰) is the smallest pulse modeled here to reasonably reproduce observations of the extent of seafl oor CaCO 3 dis- solution. This pulse could not have been solely the result of methane hydrate destabilization, suggesting that additional sources of CO 2 such as volcanic CO 2 , the oxidation of sedimentary organic carbon, or thermogenic methane must also have contributed. Observed contrasts in dissolution intensity between Atlantic and Pacifi c sites are reproduced in the model by reduc- ing bioturbation in the Atlantic during the event, simulating a potential consequence of the spread of low-oxygen bottom waters.