Trace metals such as zinc (Zn) and cadmium (Cd) are oceanographically important elements, both as micronutrients for phytoplankton and as tracers of biology in the modern and ancient ocean. The broad strokes of their distribution have been understood since they were first measured accurately in seawater decades ago, both are obviously drawn down in the surface ocean by phytoplankton and regenerated in the deep ocean. And yet, for just as long there have been mysteries about the details of Zn and Cd cycling. Why does Zn have a distribution which looks like silicate, if it occurs in the soft tissues of plankton instead of in hard silicate frustules? Why is there a slight ‘kink’ in the relationship between Cd and phosphorous at low concentrations, when their deep-ocean distribution is nearly linear? Two recent advances provide insight into such questions. GEOTRACES cruises over the past few years have added thousands of measurements of Zn and Cd concentrations and stable isotopes in the global ocean, while new Ocean Circulation Inverse Models (OCIMs) provide a modeling framework to quickly test various hypotheses about the biogeochemical modeling of these elements. The talk will include party favors, and an explanation of why the answers to the questions posed above are K, and alpha, respectively!
Simon J. Lock,
Pressure is a key parameter in the physics and chemistry of planet formation and evolution. Previous studies have erroneously assumed that internal pressures monotonically increase with the mass of a body. Using smoothed particle hydrodynamics and potential field method calculations, we demonstrate that the hot, rapidly-rotating bodies produced by giant impacts can have much lower internal pressures than cool, slowly-rotating planets of the same mass. Pressures subsequently increase due to thermal and rotational evolution of the body. Using the Moon-forming giant impact as an example, we show that the internal pressures after the collision could have been less than half that in present-day Earth. The current pressure profile was not established until Earth cooled and the Moon receded, a process that may take up to 10s Myr after the last giant impact. Our work defines a new paradigm for pressure evolution during accretion of terrestrial planets: stochastic changes driven by impacts.
Methane emitted from wetlands, Earth’s largest natural source, comes primarily from biological methanogenesis (MG) by groups of archaea, and is the product of the last step of organic matter degradation. In anoxic marine sediments, methane can be efficiently metabolized by anaerobic oxidation of methane (AOM), whereby a microbial consortium of methanotrophic archaea and sulfate-reducing bacteria oxidize methane to bicarbonate in a zone called the sulfate-methane transition zone (SMTZ). Above the SMTZ, organoclastic sulfate reduction tends to thermodynamically suppress MG when in competition for hydrogen and acetate. But MG can persist with non-competitive substrates, such as methanol and methylated amines. Methane produced within the sulfate reduction zone has the potential to directly fuel AOM, leading to a "cryptic methane cycling" above the SMTZ. Here we will present our preliminary and ongoing research on the direct relationship between MG and AOM above the SMTZ in the Carpentaria Salt Marsh Reserve, a coastal wetland located south of Santa Barbara, CA, USA. Sediment push cores were collected from a shallow hypersaline pool (~130 PSU, 72 mM sulfate). The cores had a black, sulfidic top layer (0-8 cm) and a brownish bottom layer (8-16 cm). Sediment samples were subjected to whole-core and batch incubations, radioisotope labelling, gas chromatography, and porewater analytics.Results from radiotracer incubations of sediment revealed the activity of AOM, as well as sulfate reduction within the first 4 cm of the sediment. Below 4 cm, AOM activity continued, while sulfate reduction was absent despite 67 mM sulfate.We therefore postulate that AOM in the bottom layer was coupled to the reduction of iron, which was supported by the brownish colouring of the sediment, indicating the presence of iron oxides. The addition of mono-methylamine and methanol resulted in distinct methane production in both sediment layers, pointing to the presence of a methanogenic community that was utilizing these non-competitive substrates. The addition of molybdate, a sulfate reducer inhibitor, triggered methane production in the top layer in which also sulfate reduction was detected, but at a much lower rate compared to the mono-methylamine and methanol additions. We will further present data from time-series labelling experiments with 14C-methylamine, which were designed to follow cryptic methane cycling from the methanogenic conversion of methylamine to methane, to the methanotrophic oxidation of methane to CO2.
Heather Kirkpatrick, and Jeff T. Osterhout,