Jess Adkins - Academia.edu (original) (raw)

Papers by Jess Adkins

The potential applications of ancient DNA (aDNA) techniques have been realized relatively recentl... more The potential applications of ancient DNA (aDNA) techniques have been realized relatively recently, and have been revolutionized by the advent of pCR techniques in the mid 1980s. Although these techniques have been proven valuable in ancient specimens of up to 100,000 yrs old, their use in the marine realm has been largely limited to mammals and fish. Using modifications of techniques developed for skeletons of whales and mammals, we have produced a method for extracting and amplifying aDNA from sub-fossil (not embedded in rock) deep-water corals that has been successful in yielding 351 base pairs of the ITS2 region in sub-fossil Desmophyllum dianthus (Esper, 1794) and Lophelia pertusa (Linnaeus, 1758). The comparison of DNA sequences from fossil and live specimens resulted in clustering by species, demonstrating the validity of this new aDNA method. Sub-fossil scleractinian corals are readily dated using U-series techniques, and so the abundance of directly-dateable skeletons in the world's oceans, provides an extremely useful archive for investigating the interactions of environmental pressures (in particular ocean circulation, climate change) on the past distribution, and the evolution of deep-water corals across the globe.

Organic matter (OM) burial in marine sediments is a potentially important control on global clima... more Organic matter (OM) burial in marine sediments is a potentially important control on global climate and the long-term redox state of the earth's surface. Still, we have only a limited understanding of the processes that stabilize OM and facilitate its preservation in the geologic record. Abiotic reactions with (poly)sulfides can enhance the preservation potential of OM, but for this process to be significant it needs to compete with OM remineralization, the majority of which occurs before sinking particles reach the sea floor. Here we investigate whether OM sulfurization occurs within sinking particles in the Cariaco Basin, a modern sulfidic marine environment with high rates of OM burial. Proto-kerogen in sinking particles is frequently more sulfur-rich and 34 S-depleted than expectations for biomass, with a composition that is difficult to explain by mixing with resuspended or terrigenous material. Instead, it appears that sulfur is being incorporated into OM on a timescale of days in sinking particles. The flux of this abiogenic organic S from particles is equivalent to approximately two-thirds of the total amount of proto-kerogen S at 10 cm depth in underlying sediments (ODP Core 1002B); after 6000 years of more gradual sulfurization reactions, potential water column sources are still equivalent to nearly half of the total proto-kerogen S in Cariaco sediments. Water column sulfurization is most extensive during periods of upwelling and high primary productivity and appears to involve elemental S, possibly via polysulfides. This process has the potential to deliver large amounts of OM to the sediments by making it less available for remineralization, generating OM-rich deposits. It represents a potentially dynamic sink in the global carbon cycle that can respond to changes in environmental conditions, including the size and intensity of O 2 -depleted environments. Water column OM sulfurization could also have played a more significant role in the carbon cycle during ocean anoxic events, for example during the Cretaceous.

Previous results from deep-sea pore fluid data demonstrate that the glacial deep ocean was filled... more Previous results from deep-sea pore fluid data demonstrate that the glacial deep ocean was filled with salty, cold water from the South. This salinity stratification of the ocean allows for the possible accumulation of geothermal heat in the deep-sea and could result in a water column with cold fresh water on top of warm salty water and with a corresponding increase in potential energy. For an idealized 4000 dbar two-layer water column, we calculate that there are 106J/m2(10 6 J/m 2 (106J/m2(0.2 J/kg) of potential energy available when a 0.4 psu salinity contrast is balanced by a $2 1C temperature difference. This salt-based storage of heat at depth is analogous to Convectively Available Potential Energy (CAPE) in the atmosphere. The ''thermobaric effect'' in the seawater equation of state can cause this potential energy to be released catastrophically. Because deep ocean stratification was dominated by salinity at the Last Glacial Maximum (LGM), the glacial climate is more sensitive to charging this ''thermobaric capacitor'' and can plausibly explain many aspects of the record of rapid climate change. Our mechanism could account for the grouping of Dansgaard/Oeschger events into Bond Cycles and for the different patterns of warming observed in ice cores from separate hemispheres. r

Reactions between reduced inorganic sulfur and organic compounds are thought to be important for ... more Reactions between reduced inorganic sulfur and organic compounds are thought to be important for the preservation of organic matter (OM) in sediments, but the sulfurization process is poorly understood. Sulfur isotopes are potentially useful tracers of sulfurization reactions, which often occur in the presence of a strong porewater isotopic gradient driven by microbial sulfate reduction. Prior studies of bulk sedimentary OM indicate that sulfurized products are 34 S-enriched relative to coexisting sulfide, and experiments have produced 34 S-enriched organosulfur compounds. However, analytical limitations have prevented the relationship from being tested at the molecular level in natural environments. Here we apply a new method, coupled gas chromatography -inductively coupled plasma mass spectrometry, to measure the compound-specific sulfur isotopic compositions of volatile organosulfur compounds over a 6 m core of anoxic Cariaco Basin sediments. In contrast to current conceptual models, nearly all extractable organosulfur compounds were substantially depleted in 34 S relative to coexisting kerogen and porewater sulfide. We hypothesize that this 34 S depletion is due to a normal kinetic isotope effect during the initial formation of a carbon-sulfur bond and that the source of sulfur in this relatively irreversible reaction is most likely the bisulfide anion in sedimentary porewater. The 34 S-depleted products of irreversible bisulfide addition alone cannot explain the isotopic composition of total extractable or residual OM. Therefore, at least two different sulfurization pathways must operate in the Cariaco Basin, generating isotopically distinct products. Compound-specific sulfur isotope analysis thus provides new insights into the timescales and mechanisms of OM sulfurization.

Mass-independent fractionation of sulfur isotopes (reported as D 33 S) recorded in Archean sedime... more Mass-independent fractionation of sulfur isotopes (reported as D 33 S) recorded in Archean sedimentary rocks helps to constrain the composition of Earth's early atmosphere and the timing of the rise of oxygen~2.4 billion years ago. Although current hypotheses predict uniformly negative D 33 S for Archean seawater sulfate, this remains untested through the vast majority of Archean time. We applied x-ray absorption spectroscopy to investigate the low sulfate content of particularly well-preserved Neoarchean carbonates and mass spectrometry to measure their D 33 S signatures. We report unexpected, large, widespread positive D 33 S values from stratigraphic sections capturing over 70 million years and diverse depositional environments. Combined with the pyrite record, these results show that sulfate does not carry the expected negative D 33 S from sulfur mass-independent fractionation in the Neoarchean atmosphere.

1] We investigate stalagmite trace metal ratios and carbon isotopic composition ( 13 C) as potent... more 1] We investigate stalagmite trace metal ratios and carbon isotopic composition ( 13 C) as potential paleoclimate proxies by comparing cave dripwaters, stalagmites, and bedrock composition from Gunung Mulu and Gunung Buda National Parks in northern Borneo, a tropical rainforest karst site. Three year long, biweekly time series of dripwater Mg/Ca, Sr/Ca, and 13 C from several drips at our site are not correlated with rainfall variability, indicative of a relatively weak relationship between hydroclimate and dripwater geochemistry at our site. However, combining all of the dripwater geochemical data gathered over four field trips to our site (N > 300 samples), we find that drips with highly variable Mg[Sr]/Ca have relatively invariable 18 O values close to the mean. We hypothesize that increased residence times translate into reduced variance in dripwater 18 O through mixing in the epikarst as well as increased Mg[Sr]/Ca values through increased calcite precipitation in the epikarst. Mg/Ca, Sr/Ca, and 13 C time series from three overlapping stalagmites that grew over the last 27 kyrs are characterized by strong centennial-scale variations, and bear little resemblance to previously published, well-reproduced 18 O time series from the same stalagmites. The only shared signal among the three stalagmites' geochemical time series is a relative decrease of 1% in 13 C from the Last Glacial Maximum to the Holocene, consistent with a transition from savannah (C4) to rainforest (C3) conditions documented in nearby records. Taken together, our study indicates that stalagmite Mg[Sr]/Ca ratios are poor indicators of hydroclimate conditions at our site, while stalagmite 13 C exhibits some reproducible signals on glacialinterglacial timescales. (2013), Trace metal and carbon isotopic variations in cave dripwater and stalagmite geochemistry from northern

We have developed a highly sensitive and robust method for the analysis of δ 34 S in individual o... more We have developed a highly sensitive and robust method for the analysis of δ 34 S in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O 2 + is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard-sample bracketing with peaks of SF 6 reference gas. The precision of measured δ 34 S values approaches 0.1‰ for analytes containing >40 pmol S and is better than 0.5‰ for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3‰. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in δ 34 S and demonstrate the robustness and precision of the method in complex environmental samples.

Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions ... more Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions that knit together the carbon, sulfur, and iron cycles. From a 2.1-m sediment core collected in the center of the basin, we present high-resolution profiles of the concentrations and isotopic compositions of all the major species in this system: sulfate, sulfide ( P H 2 S), elemental sulfur (S 0 ), pyrite, extractable organic sulfur (OS), proto-kerogen S, total organic and dissolved inorganic carbon, and total and reducible iron. Below 10 cm depth, the core is characterized by low apparent sulfate reduction rates (<0.01 mM/yr) except near the sulfate-methane transition zone. Surprisingly, pyrite forming in shallow sediments is $30‰ more 34 S-depleted than coexisting P H 2 S in porewater. S 0 has the same strongly 34 S-depleted composition as pyrite where it forms near the sediment-water interface, though not at depth. This pattern is not easily explained by conventional hypotheses in which sedimentary pyrite derives from abiotic reactions with porewater P H 2 S or from the products of S 0 disproportionation. Instead, we propose that pyrite formation in this environment occurs within sulfate reducing microbial aggregates or biofilms, where it reflects the isotopic composition of the immediate products of bacterial sulfate reduction. Porewater P H 2 S in Santa Barbara Basin may be more 34 S-enriched than pyrite due to equilibration with relatively 34 S-enriched OS. The difference between OS and pyrite d 34 S values would then reflect the balance between microbial sulfide formation and the abundance of exchangeable OS. Both OS and pyrite d 34 S records thus have the potential to provide valuable information about biogeochemical cycles and redox structure in sedimentary paleoenvironments.

Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate th... more Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is a mid-depth maximum in radiocarbon age during the Last Glacial Maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation. This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2-D, residual-mean dynamical model of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (~5°of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45°S by 600 m and shoals the depth of North Atlantic Deep Water convection at 50°N by 2500 m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45°S by 40‰, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM.

The stable isotope composition of radiogenic and natural elements provides a powerful tool for un... more The stable isotope composition of radiogenic and natural elements provides a powerful tool for unraveling element sources and biogeochemical processes in the marine environment. Depending on the element, trace element isotope ratios can (1) narrow possible sources of the element in a sample and/or (given a temporal history boundary condition) con-strain the time the when the element departed the ocean surface (e.g., Pb, Nd),

Coupled radiocarbon and thorium-230 dates from benthic coral species reveal that the ventilation ... more Coupled radiocarbon and thorium-230 dates from benthic coral species reveal that the ventilation rate of the North Atlantic upper deep water varied greatly during the last deglaciation. Radiocarbon ages in several corals of the same age, 15.41 Ϯ 0.17 thousand years, and nearly the same depth, 1800 meters, in the western North Atlantic Ocean increased by as much as 670 years during the 30-to 160-year life spans of the samples. Cadmium/calcium ratios in one coral imply that the nutrient content of these deep waters also increased. Our data show that the deep ocean changed on decadal-centennial time scales during rapid changes in the surface ocean and the atmosphere.

Speleothem oxygen isotopes (δ 18 O) are often used to reconstruct past rainfall δ 18 O variabilit... more Speleothem oxygen isotopes (δ 18 O) are often used to reconstruct past rainfall δ 18 O variability, and thereby hydroclimate changes, in many regions of the world. However, poor constraints on the karst hydrological processes that transform rainfall signals into cave dripwater add significant uncertainty to interpretations of speleothem-based reconstructions. Here we present several 6.5 year, biweekly dripwater δ 18 O time series from northern Borneo and compare them to local rainfall δ 18 O variability. We demonstrate that vadose water mixing is the primary rainfall-to-dripwater transformation process at our site, where dripwater δ 18 O reflects amount-weighted rainfall δ 18 O integrated over the previous 3-10 months. We document large interannual dripwater δ 18 O variability related to the El Niño-Southern Oscillation (ENSO), with amplitudes inversely correlated to dripwater residence times. According to a simple stalagmite forward model, asymmetrical ENSO extremes produce significant offsets in stalagmite δ 18 O time series given different dripwater residence times. Our study highlights the utility of generating multiyear, paired time series of rainfall and dripwater δ 18 O to aid interpretations of stalagmite δ 18 O reconstructions. MOERMAN ET AL. (2014), Transformation of ENSO-related rainwater to dripwater δ 18 O variability by vadose water mixing, Geophys. Res. Lett., 41, 7907-7915,

Deep-sea corals are a promising new archive of paleoclimate. Coupled radiocarbon and U-series dat... more Deep-sea corals are a promising new archive of paleoclimate. Coupled radiocarbon and U-series dates allow 14 C to be used as a tracer of ocean circulation rate in the same manner as it is used in the modern ocean. Diagenetic alteration of coral skeletons on the seafloor requires a thorough cleaning of contaminating phases of carbon. In addition, 10% of the coral must be chemically leached prior to dissolution to remove adsorbed modern CO 2 . A survey of modern samples from the full 14 C gradient in the deep ocean demonstrates that the coralline CaCO 3 records the radiocarbon value of the dissolved inorganic carbon.

We use pore fluid measurements of the chloride concentration and the oxygen isotopic composition ... more We use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from Ocean Drilling Program cores to reconstruct salinity and temperature of the deep ocean during the Last Glacial Maximum (LGM). Our data show that the temperatures of the deep Pacific, Southern, and Atlantic oceans during the LGM were relatively homogeneous and within error of the freezing point of seawater at the ocean's surface. Our chloride data show that the glacial stratification was dominated by salinity variations, in contrast with the modern ocean, for which temperature plays a primary role. During the LGM the Southern Ocean contained the saltiest water in the deep ocean. This reversal of the modern salinity contrast between the North and South Atlantic implies that the freshwater budget at the poles must have been quite different. A strict conversion of mean salinity at the LGM to equivalent sea-level change yields a value in excess of 140 meters. However, the storage of fresh water in ice shelves and/or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level.

1] Using a high-resolution 230 Th normalized record of sediment flux, we document the deglacial a... more 1] Using a high-resolution 230 Th normalized record of sediment flux, we document the deglacial and Holocene history of North African aridity and coastal upwelling at Ocean Drilling Program Hole 658C. At both the end of the Younger Dryas and after the 8.2 ka event, there are significant drops in terrigenous accumulation at our site, indicating an increase in the monsoon moisture flux over Africa at this time. At 5.5 ka, there is an abrupt end to the ''African humid period'' and a return to stronger upwelling conditions. For carbonate and opal fluxes the 230 Th normalization completely changes the shape of each record based on percentage variations alone. This site is a clear example of how variations in one sediment component can obscure changes in the others, and it demonstrates the need for radionuclide measurements more generally in paleoceanography. By taking our new records and a large amount of previous data from this site we conclude that increases in African moisture are tightly coupled to decreases in coastal upwelling intensity.

Compound-specific analyses of the 34 S/ 32 S isotope ratios of individual organosulfur compounds ... more Compound-specific analyses of the 34 S/ 32 S isotope ratios of individual organosulfur compounds in Upper Jurassic oil and condensate samples from the Smackover Fm. reveal differences of up to $50& between compounds. There is a clear distinction between oils altered by thermochemical sulfate reduction (TSR) versus those that are not. Oils that did experience TSR exhibit significant 34 S enrichment of benzothiophenes (BTs) compared to dibenzothiophenes (DBTs), while in unaltered oils these compounds have similar isotopic compositions. The d 34 S values of BTs are close to those of sulfate-bearing evaporites of the Smackover Fm., whereas the d 34 S values of DBTs are spread over a wider range and gradually approach those of the BTs.