nai

Project Progress

In Hayes’ group, Alex Sessions has continued work on hydrogen-isotopic biogeochemistry. During the period covered by this report, we have prepared a manuscript that deals with the “exchange” of hydrogen atoms between organic compounds and water (or surface-bound hydroxyl groups). Measurements completed in previous years have shown that the isotopic composition of “non-exchangeable” H (for example, H bound to C in hydrocarbons) can change much more rapidly than expected. The phenomenon has been traced to processes different from those involved in classical exchange reactions. Examples include carbon-skeletal rearrangements and stereochemical inversions in which the hydrogen inventory of a molecule does not change but in which C-H bonds are broken and remade. It has been necessary to erect a new “taxonomy” of processes that can affect D/H ratios in organic molecules.

Hayes, Sylva, Hinrichs, and Summons continued reconnaissance analyses of lipids from the Lost City hydrothermal vent system. The system is of interest because the vent fluids are rich in H₂ and may thus resemble settings on the primitive Earth. The samples were provided by Prof. Deborah Kelley of the University of Washington and had been collected in the course of only two Alvin dives at the site in 2000. Quantities of extractable lipids proved to be too low to allow detailed molecular and isotopic analyses. Accordingly, Hayes joined this year’s much longer Atlantis/Alvin cruise to the site (18 dives, 20 days on station). Together with Alex Bradley, a graduate student now beginning studies with Prof. Summons, he made shipboard survey analyses that identified high-lipid materials and that resulted in the focused collection of many kilograms of suitable samples.

Hayes and Summons continued collaborations with Dan Rothman, a specialist in non-linear modeling techniques and a member of the faculty at MIT. Work has focused on the carbon isotopic records (both carbonate and organic) from the late Proterozoic (see highlights below and bridge-fund proposal). Our research explores two broad themes. The first is the chemical and isotopic characterization and biogeochemical significance of lipids from cultured microbes and environmental samples with well characterized microbiota. The second is a search for fossil analogues in ancient sedimentary environments (0.5 to 2.7Ga).

In collaboration with Linda Jahnke and Victoria Orphan (NASA Ames), Helen Sturt at WHOI, Kai Hinrichs (Univ. Bremen) and a research group at the University of Regensburg, Germany (Dr Robert Huber, Dr Harald Huber and their graduate students), Roger Summons has characterized the lipids of new isolates of extremophilic bacteria (Aquificales sp., Thermovibrio ruber and Thiovolum) and archaea (e.g. Ignicoccus sp., Nannoarchaeum equitans, Methanococcus sp.) using a combination of LC-MS-MS to determine intact lipids degradation to precisely identify hydrocarbon skeletons.

On the ancient sediment side, Summons is studying two important and well preserved sequences in Oman and Australia for evidence of environmental change through the Proterozoic Eon. In studies that draw on his earlier hydrocarbon work in the 1.7-1.4Ga McArthur Basin of Northern Australia, Summons and colleagues have discovered evidence for photoic zone anoxia by way of biomarkers diagnostic for the green and purple sulfur bacteria (Chlorobiaceae and Chromatiaceae) in the Barney Creek Formation. They are now evaluating the stratigraphic and lithological relationships between these markers and those of oxygenic phototrophs in order to develop a model of the redox evolution of the basin system. In studies of the latest Neoproterozoic sequence from the Oman Salt Basin, Summons is studying chemostratigraphic relationships and characterizing organofacies (eg. stromatolitic, thrombolitic and deep basin sapropel) in rocks that bridge the Cambrian-Precambrian boundary, with the aim of discriminating global secular and local environmental effects.

Dan Schrag has been studying rates of methanogenesis in sediments from the South American continental margin. Vigorous upwelling off the west coast of South America leads to the deposition of organic rich sediments, creating chemical potential gradients that are host to diverse microbial communities. Ocean Drilling Program Leg 201 drilled several sites from this region last year. Degradation of organic matter generates extremely high alkalinity as well as large concentrations of dissolved ammonium and phosphate. High alkalinity drives the shallow precipitation of authigenic carbonate, resulting in depletions of dissolved Ca that masks that released by calcite dissolution (as indicated by increases with depth in dissolved Sr). Dolomite precipitation causes significant negative excursions of dissolved Mg from background downcore trends that are due to uptake during clay alteration. These excursions are coincident with sharply defined minima in Ca. Isotopic analysis of dissolved inorganic carbon carried out by our laboratory shows a pattern characteristic of sites dominated by methanogenesis. δ¹³C values decrease sharply with depth in the first 10 to 50 meters, reaching values as low as -30 per mil, indicating organic matter oxidation as well as methanotrophy. Values increase below the zone where sulfate disappears, reaching a plateau at values between 0 and +15 per mil. In some cases, the sulfate content of the pore fluid increases again, because sulfate rich brine flows through the basaltic basement rock. Schrag is currently working with numerical models to calculate the rates of methanogenesis and methanotrophy to begin to ask questions about what limits the biological activity in these environments.