Isotopic Constraints on Nitrogen Transformation Rates in the Deep Sedimentary Marine Biosphere

Buchwald, Carolyn; Homola, K.; Spivack, Arthur J.; Estes, Emily R.; Murray, Richard W; Wankel, Scott D.

doi:10.1029/2018gb005948

Cited by 17 publications

(13 citation statements)

References 73 publications

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“…The oxygen profile is concave since oxygen is consumed over the entire upper oxic zone (e.g. Froelich et al, 1979;Cai, and Sayles, 1996;Wenzhöfer and Gludd, 2002). The cores analysed in this study show the typical features of abyssal, carbonstarved ocean sediments with sufficient deep-water ventila-Figure 3.…”

Section: Discussionmentioning

confidence: 88%

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific

et al. 2021

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Abstract. Exchange of dissolved substances at the sediment–water interface provides an important link between the short-term and long-term geochemical cycles in the ocean. A second, as yet poorly understood sediment–water exchange is supported by low-temperature circulation of seawater through the oceanic basement underneath the sediments. From the basement, upwards diffusing oxygen and other dissolved species modify the sediment, whereas reaction products diffuse from the sediment down into the basement where they are transported by the basement fluid and released to the ocean. Here, we investigate the impact of this “second” route with respect to transport, release and consumption of oxygen, nitrate, manganese, nickel and cobalt on the basis of sediment cores retrieved from the Clarion Clipperton Zone (CCZ) in the equatorial Pacific Ocean. We show that in this abyssal ocean region characterised by low organic carbon burial and sedimentation rates vast areas exist where the downward- and upward-directed diffusive fluxes of oxygen meet so that the sediments are oxic throughout. This is especially the case where sediments are thin or in the proximity of faults. Oxygen diffusing upward from the basaltic crust into the sediment contributes to the degradation of sedimentary organic matter. Where the sediments are entirely oxic, nitrate produced in the upper sediment by nitrification is lost both by upward diffusion into the bottom water and by downward diffusion into the fluids circulating within the basement. Where the oxygen profiles do not meet, they are separated by a suboxic sediment interval characterised by Mn2+ in the porewater. Where porewater Mn2+ in the suboxic zones remains low, nitrate consumption is low and the sediment continues to deliver nitrate to the ocean bottom waters and basement fluid. We observe that at elevated porewater manganese concentrations, nitrate consumption exceeds production and nitrate diffuses from the basement fluid into the sediment. Within the suboxic zone, not only manganese but also cobalt and nickel are released into the porewater by reduction of Mn oxides, diffusing towards the oxic–suboxic fronts above and below where they precipitate, effectively removing these metals from the suboxic zone and concentrating them at the two oxic–suboxic redox boundaries. We show that not only do diffusive fluxes in the top part of deep-sea sediments modify the geochemical composition over time but also diffusive fluxes of dissolved constituents from the basement into the bottom layers of the sediment. Hence, the palaeoceanographic interpretation of sedimentary layers should carefully consider such deep secondary modifications in order to prevent the misinterpretation of primary signatures.

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Section: Discussionmentioning

confidence: 88%

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific

et al. 2021

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“…These occurrences can be explained by original deposition of much higher oxidized metal concentrations or much lower organic matter concentration in deep metal-reducing zones than in shallower deposits. Second, isotopic data, transcriptomic data and radiotracer data have respectively been interpreted as evidence of NO 2 − oxidation 116 , NO 3 − reduction 25 and SO 4 2− reduction 117 in sediment deep beneath the last occurrences of measurable O 2 , NO 3 − and SO 4 2− . These processes can only occur at these depths if cryptic processes, such as reduction of H 2 O 2 from water radiolysis 38 , sustain in situ production of the necessary electron acceptors (O 2 , NO 3 − , SO 4 2− ).…”

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confidence: 99%

Subseafloor life and its biogeochemical impacts

et al. 2019

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Subseafloor microbial activities are central to Earth’s biogeochemical cycles. They control Earth’s surface oxidation and major aspects of ocean chemistry. They affect climate on long timescales and play major roles in forming and destroying economic resources. In this review, we evaluate present understanding of subseafloor microbes and their activities, identify research gaps, and recommend approaches to filling those gaps. Our synthesis suggests that chemical diffusion rates and reaction affinities play a primary role in controlling rates of subseafloor activities. Fundamental aspects of subseafloor communities, including features that enable their persistence at low catabolic rates for millions of years, remain unknown.

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“…Samples from 2010–2012 were measured according to Rafter & Sigman, . Those from 2013–2015 were measured according to Buchwald et al, . Briefly, sample N 2 O was purified using a customized purge and trap system and analyzed on a continuous flow IsoPrime 100 isotope ratio mass spectrometer.…”

Section: Methodsmentioning

confidence: 99%

Recent Increases in Water Column Denitrification in the Seasonally Suboxic Bottom Waters of the Santa Barbara Basin

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Rafter

Stephens

et al. 2019

Geophysical Research Letters

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Denitrification in the anoxic sediments of the Santa Barbara Basin has been well documented in the historic and modern record, but the regulation of and frequency with which denitrification occurs in the overlying water column are less understood. Since 2004, the magnitude and speciation of redox active nitrogen species in bottom waters have changed markedly. Most notable are periods of decreased nitrate and increased nitrite concentrations. Here we examine these changes in nitrogen cycling as recorded by the stable isotopes of dissolved nitrate from 2010–2016. When compared to previous studies, our data identify an increase in water column denitrification in the bottom waters of the basin. Observations from inside the basin as well as data from the wider California Current Ecosystem implicate a long‐term trend of decreasing oxygen concentrations as the driver for these observed changes, with ramifications for local benthic communities and regional nitrogen loss.

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Isotopic Constraints on Nitrogen Transformation Rates in the Deep Sedimentary Marine Biosphere

Cited by 17 publications

References 73 publications

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the northeast Pacific

Subseafloor life and its biogeochemical impacts

Recent Increases in Water Column Denitrification in the Seasonally Suboxic Bottom Waters of the Santa Barbara Basin

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