Showing papers by "Ellen Thomas published in 2017"

Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum

Abstract: The Palaeocene–Eocene Thermal Maximum1,2 (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates3. However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming1,3,4,5. Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data—a proxy for seawater pH—that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event6,7. We find strong evidence for a much larger (more than 10,000 petagrams)—and, on average, isotopically heavier—carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province10,11, rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system12.

A model for the decrease in amplitude of carbon isotope excursions across the Phanerozoic

Abstract: The geological cycling of carbon ties together the ocean-atmosphere carbon pool, Earth9s biosphere, and Earth9s sedimentary reservoirs. Perturbations to this coupled system are recorded in the carbon-isotopic (δ13C) composition of marine carbonates. Large amplitude δ13C excursions are typically treated as individual events and interpreted accordingly. However, a recent compilation of Phanerozoic carbon isotopic data reveals that δ13C excursions are a ubiquitous feature of the geologic record, and thus should be considered in concert. Analysis indicates that Phanerozoic carbon isotope excursions, as a whole, have characteristic durations of 0.5 to 10 M.yr. and exhibit declining amplitude over time. These commonalities suggest a shared underlying control. Here we demonstrate that sinusoidal modulation of the sensitivity of organic carbon and phosphate burial in a simple numerical model of the geologic carbon cycle results in large, asymmetric δ13C oscillations that exhibit their largest amplitudes in the 0.5 to 10 M.yr. period range. As anoxia is known to strongly modulate the C:P burial ratio of organic matter in sediments, we propose that sea-level oscillations were the primary source of sinusoidal modulation for the geologic carbon cycle, and that their degree of influence on the carbon cycle was determined by the state of oxygenation of bottom waters overlying the continental shelves. When oxygen minimum zones (OMZs) were large, shallow, and prone to expansion, sea-level changes would have had the capacity to drive large changes in the areal extent of OMZs in contact with the sea-floor, resulting in strong leverage on the burial sensitivity of organic carbon and phosphate, and thus on δ13C. Progressive oxygenation of the oceans, which was facilitated by biological innovations, resulted in a decline in the amplitude of δ13C excursions over the Phanerozoic, and the biogeochemical stabilization of the Earth System.

A tale of two lakes: the Newberry Volcano twin crater lakes, Oregon, USA

Abstract: Abstract Newberry Volcano in Oregon, USA, has two small crater lakes inside its caldera: East Lake and Paulina Lake. The 50–80 m deep lakes differ in morphology, water chemistry and sediment composition, although separated only by a narrow volcanic ridge. East Lake is a terminal lake with gaseous geothermal inputs, whereas Paulina Lake has an outlet (Paulina Creek) and subaqueous, high-alkalinity hot springs. The sediment in both lakes is organic-rich and mainly consists of diatom frustules (SiO2) with some volcanic ash, accumulating at sedimentation rates of 1.5–2 mm a−1 (210Pb dated). In Paulina Lake the sediment has up to 14% Fe2O3 of hydrothermal origin and 250 ppm As. Sediment in East Lake is Fe-poor but has up to 4 ppm Hg, and fish are also Hg-rich. Both lakes host productive ecosystems, with primary producers using geothermal CO2, P and Si, and nitrogen fixed by cyanobacteria (Nostoc sp.). Water budgets and water residence times were calculated from stable isotope budgets. East Lake has a steep vertical δ13C (dissolved inorganic carbon, DIC) gradient, with surface δ13C values of up to 5.5‰, largely due to diffusional CO2 losses and photosynthetic carbon withdrawal. Paulina Lake is better mixed, has a lower organic productivity and limited surface CO2 evasion; its δ13C (DIC) gradient is small. We theorize that the lakes are fed by different geothermal components as a result of phase separation below East Lake. The gas component (CO2, H2S, Hg) enters East Lake, as indicated by the rising bubble trains in the lake. A residual fluid, depleted in Hg but rich in Si, Fe, carbonate, P and As, enters Paulina Lake. The presence of highly toxic components and the gas-charging of East Lake present natural hazards, which may change when new volcanic activity is initiated, and thus should be monitored.

Upper Maastrichtian-Eocene benthic foraminiferal biofacies of the Brazilian margin, western South Atlantic

Abstract: Benthic foraminiferal biofacies were delimited for the upper Maastichtian through upper Eocene of Brazilian marginal basins (Sergipe-Alagoas, Mucuri, Campos, Santos and Pelotas) and two DSDP Sites and 20C of the western South Atlantic. The biofacies were determined based on the benthic assemblages and associated parameters, including percentage of planktic foraminifera (% lithology, and percentage of radiolarians (% rads). The biofacies show basin-to-basin differences, and primarily distinguished by the taxon percentage and the dominant three or species. Biofacies A is composed of 100% calcareous taxa and dominates in the Eocene. Biofacies B up to 10% agglutinated taxa and occurs from the middle Paleocene through the upper Eocene. Biofacies has 11% to 25% agglutinated taxa and is present from the upper Maastrichtian through the upper Biofacies D contains a balanced percentage of calcareous and agglutinated taxa (~50% each), and is associated with Biofacies E in the marginal basins. Biofacies E is dominated by agglutinated taxa, tubular forms (Bathysiphon, Nothia, Rhizammina, This biofacies correlates with the called “flyschtype” biofacies of Berggren and Gradstein (1981), and occurs exclusively in the basins from the Maastrichtian through upper Eocene, although it dominates in the Paleocene. The distribution reveals distinct environmental settings as the Brazilian margin built outwards in response tectonic activity and increased terrigenous input. The biofacies record a deep-water setting close to or the calcite compensation depth (CCD) during the along the entire eastern margin. Progradation of the shelf and shoaling of the slope during the Eocene probably is the reason for the abrupt change from agglutinant-rich biofacies (E and D) to calcareous-rich biofacies (A, and C) in the early Eocene. The Campos Basin continued to record abyssal to lower bathyal while the Sergipe-Alagoas and Mucuri basins shoaled to neritic palaeodepths by the late Eocene. Changes relative sea level, including a global sea level fall in the late Paleocene followed by global sea level rise the early Eocene, as well as changes in the position of the CCD along the Brazilian margin affected development of foraminiferal biofacies in the marginal basins. The distal DSDP sites were at palaeodepths than the sites in the marginal basins, but were not below the CCD during the Eocene and biofacies were dominantly calcareous. We conclude that the CCD was shallower along productive Brazilian continental than in the pelagic areas due to the greater flux of organic