Author
Richard A. Jahnke
Other affiliations: Scripps Institution of Oceanography
Bio: Richard A. Jahnke is an academic researcher from Skidaway Institute of Oceanography. The author has contributed to research in topics: Benthic zone & Total organic carbon. The author has an hindex of 36, co-authored 50 publications receiving 5758 citations. Previous affiliations of Richard A. Jahnke include Scripps Institution of Oceanography.
Papers
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TL;DR: In this article, the cycling of Fe, Mn, Ni, Co, Cu, Cr, V, and Mo during early diagenesis was investigated in sediments from five different depositional environments in the California Borderland.
649 citations
TL;DR: In this article, a positive feedback linking ocean anoxia, enhanced benthic phosphorus regeneration, and marine productivity is proposed, which suggests that O2 depletion in coastal regions caused by eutrophication may enhance P regeneration from sediments, thereby providing additional P necessary for increased biological productivity.
446 citations
TL;DR: The magnitude and distribution of the particulate organic carbon (POC) rain rate to the seafloor in the Atlantic, Pacific and Indian Ocean basins between 61°N and 61°S has been estimated from benthic oxygen flux estimates (for water depths ≥ 1000 m only).
Abstract: The magnitude and distribution of the particulate organic carbon (POC) rain rate to the seafloor in the Atlantic, Pacific and Indian Ocean basins between 61°N and 61°S has been estimated from benthic oxygen flux estimates (for water depths ≥ 1000 m only) The calculation utilizes the extensive data sets of sedimentary organic carbon, CaCO3, and accumulation rate to extrapolate between individual benthic flux measurement sites using an empirically-derived correlation between the seafloor oxygen flux and these parameters The POC flux through the 1000 m depth horizon was then estimated from published correlations between sediment trap-determined fluxes and water depth Total oxygen utilization in the deep ocean is estimated to be 12×1014 mol O2 yr−1, a value that agrees well with previous estimates which were based on surface water primary productivity, sediment trap, and depth relationships and with deep water respiration rates estimated from apparant oxygen utilization (AOU)-14C relationships On the basis of the derived global ocean flux distribution, it is concluded that (1) dissolved organic carbon (DOC) inputs are not required to account for estimated deep water respiration rates; (2) the majority of the POC input to the deep ocean occurs within 30° of the equator; (3) the proportion of primary production that reaches the deep sea does not vary greatly with latitude; (4) gyre and continental margin regions contribute roughly equally to the deep POC flux with a relatively minor contribution from the equatorial divergence region; (5) of the estimated 72×1013 mol C yr−1 of POC that sinks below the 1000 m depth horizon, 45% (33×1013 mol C yr−1) reaches the seafloor where it is oxidized; (6) when normalized to basin area, average deep flux rates in the Atlantic and Pacific are similar while highest rates are observed in the Indian Ocean; and (7) the results can be fully reconciled only if the benthic flux of DOC is significantly less than the benthic O2 flux
403 citations
TL;DR: Pore water chemistry (total dissolved CO 2, NH 4, NO 3, NO 2, PO 4, Si(OH) 4, Ca, Mg, Fe, Mn, SO 4, H 2 S and F, and titration alkalinity) and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20-40 cm of sediments under intense upwelling areas on the Peru continental shelf as discussed by the authors.
337 citations
TL;DR: In this article, a positive feedback between water column anoxia, enhanced benthic phosphorus regeneration, and marine productivity is proposed to explain the widespread accumulation of organic-rich marine sediments from anoxic waters observed in the geologic record.
291 citations
Cited by
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TL;DR: In this paper, a synthesis of the use of selected trace elements as proxies for reconstruction of paleoproductivity and paleoredox conditions is presented, and the combined used of U, V and Mo enrichments may allow suboxic environments to be distinguished from anoxic-euxinic ones.
2,708 citations
TL;DR: For example, in a recent paper as discussed by the authors, the authors investigated the mechanisms governing sedimentary organic matter preservation in marine sediments and found that organic preservation in the marine environment is < 0.5% efficient, and that the factors which directly determine preservation vary with depositional regime, but have in common a critical interaction between organic and inorganic materials over locally variable time scales.
2,216 citations
TL;DR: The precision of the method is better than 0.2/1000 (1 SD) at concentrations of nitrate down to 1 microM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced.
Abstract: We report a new method for measurement of the isotopic composition of nitrate (NO3-) at the natural-abundance level in both seawater and freshwater. The method is based on the isotopic analysis of nitrous oxide (N2O) generated from nitrate by denitrifying bacteria that lack N2O-reductase activity. The isotopic composition of both nitrogen and oxygen from nitrate are accessible in this way. In this first of two companion manuscripts, we describe the basic protocol and results for the nitrogen isotopes. The precision of the method is better than 0.2‰ (1 SD) at concentrations of nitrate down to 1 μM, and the nitrogen isotopic differences among various standards and samples are accurately reproduced. For samples with 1 μM nitrate or more, the blank of the method is less than 10% of the signal size, and various approaches may reduce it further.
1,562 citations
TL;DR: In this article, eight geochemical indices used for the interpretation of bottom water palaeo-oxygen concentrations for argillaceous sedimentary rocks have been calculated for a suite of Upper Jurassic mudstones drawn from the Draupne and Heather Formations of the Norwegian North Sea, and the Kimmeridge Clay Formation from onshore England.
1,551 citations
TL;DR: The current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms is examined in this paper.
Abstract: Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.
1,481 citations