Bottom water anoxia, inoceramid colonization, and benthopelagic coupling during black shale deposition on Demerara Rise (Late Cretaceous western tropical North Atlantic)

Abstract. In the ocean, sinking of particulate organic matter (POM) drives carbon export from the euphotic zone and supplies nutrition to mesopelagic communities, the feeding and degradation activities of which in turn lead to export flux attenuation. Oxygen (O2) minimum zones (OMZs) with suboxic water layers (< 5 µmol O2 kg−1) show a lower carbon flux attenuation compared to well-oxygenated waters (> 100 µmol O2 kg−1), supposedly due to reduced heterotrophic activity. This study focuses on sinking particle fluxes through hypoxic mesopelagic waters (< 60 µmol O2 kg−1); these represent ∼  100 times more ocean volume globally compared to suboxic waters, but they have less been studied. Particle export fluxes and attenuation coefficients were determined in the eastern tropical North Atlantic (ETNA) using two surface-tethered drifting sediment trap arrays with seven trapping depths located between 100 and 600 m. Data on particulate matter fluxes were fitted to the normalized power function Fz =  F100 (z∕100)−b, with F100 being the flux at a depth (z) of 100 m and b being the attenuation coefficient. Higher b values suggest stronger flux attenuation and are influenced by factors such as faster degradation at higher temperatures. In this study, b values of organic carbon fluxes varied between 0.74 and 0.80 and were in the intermediate range of previous reports, but lower than expected from seawater temperatures within the upper 500 m. During this study, highest b values were determined for fluxes of particulate hydrolyzable amino acids (PHAA), followed by particulate organic phosphorus (POP), nitrogen (PN), carbon (POC), chlorophyll a (Chl a) and transparent exopolymer particles (TEP), pointing to a sequential degradation of organic matter components during sinking. Our study suggests that in addition to O2 concentration, organic matter composition co-determines transfer efficiency through the mesopelagic. The magnitude of future carbon export fluxes may therefore also depend on how organic matter quality in the surface ocean changes under influence of warming, acidification and enhanced stratification.

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Temporal variability of air–sea CO 2 flux in the western tropical North Atlantic influenced by the Amazon River plume

Global Biogeochemical Cycles ◽

10.1029/2020gb006798 ◽

2021 ◽

Author(s):

L Mu ◽

HR Gomes ◽

SM Burns ◽

JI Goes ◽

VJ Coles ◽

...

Keyword(s):

North Atlantic ◽

Temporal Variability ◽

River Plume ◽

Amazon River ◽

Tropical North Atlantic ◽

Amazon River Plume

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Open ocean dead zones in the tropical North Atlantic Ocean

Biogeosciences ◽

10.5194/bg-12-2597-2015 ◽

2015 ◽

Vol 12 (8) ◽

pp. 2597-2605 ◽

Cited By ~ 59

Author(s):

J. Karstensen ◽

B. Fiedler ◽

F. Schütte ◽

P. Brandt ◽

A. Körtzinger ◽

...

Keyword(s):

North Atlantic ◽

North Atlantic Ocean ◽

Dead Zone ◽

Marine Ecosystem ◽

High Surface ◽

Open Ocean ◽

Depth Range ◽

Low Oxygen ◽

Dead Zones ◽

Tropical North Atlantic

Abstract. Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low (<2 μmol kg−1) oxygen environments in the open waters of the tropical North Atlantic, a region where oxygen concentration does normally not fall much below 40 μmol kg−1. The low-oxygen zones are created at shallow depth, just below the mixed layer, in the euphotic zone of cyclonic eddies and anticyclonic-modewater eddies. Both types of eddies are prone to high surface productivity. Net respiration rates for the eddies are found to be 3 to 5 times higher when compared with surrounding waters. Oxygen is lowest in the centre of the eddies, in a depth range where the swirl velocity, defining the transition between eddy and surroundings, has its maximum. It is assumed that the strong velocity at the outer rim of the eddies hampers the transport of properties across the eddies boundary and as such isolates their cores. This is supported by a remarkably stable hydrographic structure of the eddies core over periods of several months. The eddies propagate westward, at about 4 to 5 km day−1, from their generation region off the West African coast into the open ocean. High productivity and accompanying respiration, paired with sluggish exchange across the eddy boundary, create the "dead zone" inside the eddies, so far only reported for coastal areas or lakes. We observe a direct impact of the open ocean dead zones on the marine ecosystem as such that the diurnal vertical migration of zooplankton is suppressed inside the eddies.

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A quest for the biological sources of long chain alkyl diols in the western tropical North Atlantic Ocean

Biogeosciences ◽

10.5194/bg-15-5951-2018 ◽

2018 ◽

Vol 15 (19) ◽

pp. 5951-5968 ◽

Cited By ~ 11

Author(s):

Sergio Balzano ◽

Julie Lattaud ◽

Laura Villanueva ◽

Sebastiaan W. Rampen ◽

Corina P. D. Brussaard ◽

...

Keyword(s):

Atlantic Ocean ◽

Water Column ◽

North Atlantic ◽

18S Rrna ◽

North Atlantic Ocean ◽

18S Rrna Gene ◽

Rrna Gene ◽

Tropical North Atlantic ◽

Biological Sources ◽

Long Chain

Abstract. Long chain alkyl diols (LCDs) are widespread in the marine water column and sediments, but their biological sources are mostly unknown. Here we combine lipid analyses with 18S rRNA gene amplicon sequencing on suspended particulate matter (SPM) collected in the photic zone of the western tropical North Atlantic Ocean at 24 stations to infer relationships between LCDs and potential LCD producers. The C30 1,15-diol was detected in all SPM samples and accounted for >95 % of the total LCDs, while minor proportions of C28 and C30 1,13-diols, C28 and C30 1,14-diols, as well as C32 1,15-diol were found. The concentration of the C30 and C32 diols was higher in the mixed layer of the water column compared to the deep chlorophyll maximum (DCM), whereas concentrations of C28 diols were comparable. Sequencing analyses revealed extremely low contributions (≈0.1 % of the 18S rRNA gene reads) of known LCD producers, but the contributions from two taxonomic classes with which known producers are affiliated, i.e. Dictyochophyceae and Chrysophyceae, followed a trend similar to that of the concentrations of C30 and C32 diols. Statistical analyses indicated that the abundance of 4 operational taxonomic units (OTUs) of the Chrysophyceae and Dictyochophyceae, along with 23 OTUs falling into other phylogenetic groups, were weakly (r≤0.6) but significantly (p value <0.01) correlated with C30 diol concentrations. It is not clear whether some of these OTUs might indeed correspond to C28−32 diol producers or whether these correlations are just indirect and the occurrence of C30 diols and specific OTUs in the same samples might be driven by other environmental conditions. Moreover, primer mismatches were unlikely, but cannot be excluded, and the variable number of rRNA gene copies within eukaryotes might have affected the analyses leading to LCD producers being undetected or undersampled. Furthermore, based on the average LCD content measured in cultivated LCD-producing algae, the detected concentrations of LCDs in SPM are too high to be explained by the abundances of the suspected LCD-producing OTUs. This is likely explained by the slower degradation of LCDs compared to DNA in the oxic water column and suggests that some of the LCDs found here were likely to be associated with suspended debris, while the DNA from the related LCD producers had been already fully degraded. This suggests that care should be taken in constraining biological sources of relatively stable biomarker lipids by quantitative comparisons of DNA and lipid abundances.

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The distribution of neodymium isotopes and concentrations in the eastern tropical North Atlantic

Chemical Geology ◽

10.1016/j.chemgeo.2018.11.024 ◽

2019 ◽

Vol 511 ◽

pp. 265-278 ◽

Cited By ~ 4

Author(s):

Moritz Zieringer ◽

Martin Frank ◽

Roland Stumpf ◽

Ed C. Hathorne

Keyword(s):

North Atlantic ◽

Neodymium Isotopes ◽

Tropical North Atlantic

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Flow of Antarctic Bottom Water from the Vema Channel

10.21203/rs.3.rs-47589/v3 ◽

2020 ◽

Author(s):

Eugene G Morozov ◽

Dmitry I. Frey ◽

Roman Y. Tarakanov

Keyword(s):

Continental Slope ◽

Water Flow ◽

North Atlantic ◽

Bottom Water ◽

The Other ◽

Antarctic Bottom Water ◽

The North ◽

The North Atlantic ◽

Current Flows ◽

Western Wall

Abstract We analyze measurements of bottom currents and thermohaline properties of water north of the Vema Channel with the goal to find pathway continuations of Antarctic Bottom Water flow from the Vema Channel into the Brazil Basin. The analysis is based on CTD/LADCP casts north of the Vema Channel. The flow in the deep Vema Channel consists of two branches. The deepest current flows along the bottom in the center of the channel and the other branch flows above the western wall of the channel. We found two smaller channels of the northern continuation of the deeper bottom flow. These flows become weak and almost disappear at a latitude of 25°30’S. The upper current flows at a depth of 4100-4200 m along the continental slope. We traced this current up to 24°S over a distance exceeding 250 km. This branch transports bottom water that eventually fills the deep basins of the North Atlantic.

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Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

Biogeosciences Discussions ◽

10.5194/bgd-9-14291-2012 ◽

2012 ◽

Vol 9 (10) ◽

pp. 14291-14325 ◽

Cited By ~ 2

Author(s):

T. Fischer ◽

D. Banyte ◽

P. Brandt ◽

M. Dengler ◽

G. Krahmann ◽

...

Keyword(s):

North Atlantic ◽

Large Scale ◽

Oxygen Supply ◽

Water Layer ◽

Oxygen Minimum Zone ◽

Depth Interval ◽

Diapycnal Mixing ◽

Tropical North Atlantic ◽

Minimum Zone ◽

Oxygen Minimum

Abstract. The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off West Africa in the tropical North Atlantic Ocean is studied, with a focus on oxygen transport across density surfaces (diapycnal flux). The latter is obtained from a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: a large scale tracer release experiment, microstructure profiles, and shipboard acoustic current measurements plus density profiles. The average diapycnal diffusivity in the study area is 1 × 10−5 m2 s−1. No significant vertical gradient of average diapycnal diffusivities exists in the depth interval from 150 to 500 m. The diapycnal flux is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 µmol kg−1 a−1 of oxygen is supplied via diapycnal mixing, contributing about a third of the total demand. The oxygen that is contributed via diapycnal mixing originates from oxygen that has been laterally supplied within the overlying Central Water layer by advective and eddy fluxes. Due to the existence of a separate shallow oxygen minimum at about 100 m depth throughout most of the study area, there is no direct net vertical oxygen flux from the surface layer of the study area into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

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