49 research outputs found
An inverse relationship between production and export efficiency in the Southern Ocean
Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 1557–1561, doi:10.1002/grl.50219.In the past two decades, a number of studies have been carried out in the Southern Ocean to look at export production using drifting sediment traps and thorium-234 based measurements, which allows us to reexamine the validity of using the existing relationships between production, export efficiency, and temperature to derive satellite-based carbon export estimates in this region. Comparisons of in situ export rates with modeled rates indicate a two to fourfold overestimation of export production by existing models. Comprehensive analysis of in situ data indicates two major reasons for this difference: (i) in situ data indicate a trend of decreasing export efficiency with increasing production which is contrary to existing export models and (ii) the export efficiencies appear to be less sensitive to temperature in this region compared to the global estimates used in the existing models. The most important implication of these observations is that the simplest models of export, which predict increase in carbon flux with increasing surface productivity, may require additional parameters, different weighing of existing parameters, or separate algorithms for different oceanic regimes.This work was supported by NASA award
number NNX08AB48G.2013-10-2
Emerging wetlands from river diversions can sustain high denitrification rates in a Coastal Delta
Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 126(5), (2021): e2020JG006217, https://doi.org/10.1029/2020JG006217.It is assumed that to treat excess NO3− high soil organic matter content (%OM) is required to maintain high denitrification rates in natural or restored wetlands. However, this excess also represents a risk by increasing soil decomposition rates triggering peat collapse and wetland fragmentation. Here, we evaluated the role of %OM and temperature interactions controlling denitrification rates in eroding (Barataria Bay-BLC) and emerging (Wax Lake Delta-WLD) deltaic regions in coastal Louisiana using the isotope pairing (IPT) and N2:Ar techniques. We also assessed differences between total (direct denitrification + coupled nitrification-denitrification) and net (total denitrification minus nitrogen fixation) denitrification rates in benthic and wetland habitats with contrasting %OM and bulk density (BD). Sediment (benthic) and soil (wetland) cores were collected during summer, spring, and winter (2015–2016) and incubated at close to in-situ temperatures (30°C, 20°C, and 10°C, respectively). Denitrification rates were linearly correlated with temperature; maximum mean rates ranged from 40.1–124.1 μmol m−2 h−1 in the summer with lower rates (30 μM) and water temperature is >10°C. In coastal Louisiana, substrates under these regimes are represented by emergent supra-tidal flats or land created by sediment diversions under oligohaline conditions (<1 ppt).This study was supported by the NOAA-Sea Grant Program-Louisiana (Grant 2013R/E-24) to Victor H. Rivera-Monroy and Kanchan Maiti. Victor H. Rivera-Monroy was also supported by the Department of the Interior South-Central Climate Adaptation Science Center (Cooperative Agreement #G12AC00002)
Insights into Particle Formation and Remineralization Using the Short-Lived Radionuclide, Thorium-234
[1] Simple mass balance models are applied to a high resolution 234Th profile from the northwest Pacific to examine the magnitude, rate, and depth distribution of particle remineralization processes below the euphotic zone (Ez). Here, excess 234Th (234Th \u3e 238U) below the Ez is attributed to fragmentation processes that result in the conversion of sinking to non‐sinking particles. By considering particulate organic carbon (POC) to 234Th ratios on particles, we show that POC flux attenuation is larger than for 234Th, which we attribute to bacterial and zooplankton consumption of sinking POC. Three case studies are used to demonstrate how different combinations of particle fragmentation and POC respiration impact flux attenuation below the Ez. When sampled with high vertical resolution and precision, 234Th and POC/234Th ratios provide insights into both export from the Ez and the extent to which sinking particle fluxes and associated minerals are attenuated with depth
Effects of flow rates and composition of the filter, and decay/ingrowth correction factors involved with the determination of in situ particulate \u3csup\u3e210\u3c/sup\u3ePo and \u3csup\u3e210\u3c/sup\u3ePb in seawater
Accurate measurements of particulate 210Po (210Pop) and 210Pb (210Pbp) are required in the investigation of (i) partitioning of Po and Pb between particulate and dissolved phases and (ii) export estimates of carbon and other key trace metals from the euphotic zone via sinking particulate matter. Based on the intercomparison of different composition of the filter material (QMA, Supor, Millipore, and Pall GN6) and flow rates (2 to 8 L min), we show how these factors affect the measured concentrations of 210Pop and 210Pbp and their activity ratios (AR). As such, we recommend using Supor 0.4 μm filter and a flow rate of up to 8 L/min for the measurements of210Pop and 210Pbp. Furthermore, we inter-compared 210Pop and 210Pbp obtained by small-volume McLane and large-volume MULVFS pumps. The activities of 210Pop in MULVFS filter samples are somewhat higher than that of McLane filter samples, whereas the 210Pop/210Pb AR collected by McLane pump is distinctly lower, suggesting some fractionation in the collection process by the pumping systems. Likewise comparison of vertical profiles of 210Pop and 210Pbp obtained using McLane pumps by two independent research groups at the two intercalibration stations in the Pacific Ocean show quite similar values except in the mesopelagic waters, suggesting possible uneven loading and presence of larger gelatinous plankton in the filter. Finally, we append a detailed analysis of various correction factors for the accurate calculation of in situ 210Po and 210Pb. Presented results are relevant to the worldwide community that uses 210Po-210Pb as tracers in aquatic systems
Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009):1143-1167, doi:10.1016/j.dsr.2009.04.001.An extensive 234Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic
site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical
Transport in the Global Ocean) study. Total 234Th:238U activity ratios near 1.0 indicated low particle fluxes at
ALOHA, while 234Th:238U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial
variability was large at both sites- even greater than seasonal variability as reported in prior studies. This
variability in space and time confounds the use of single profiles of 234Th for sediment trap calibration
purposes. At K2, there was a decrease in export flux and increase in 234Th activities over time associated with
the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux
predictions. This variability in space and time confounds the use of single profiles of 234Th for sediment trap
calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th below the
deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at
depth that may be missed with standard sampling for 234Th and/or with sediment traps. Also, the application of
234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th ratios and here the ratio is
similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 μm at ALOHA and
>5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of
234Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column
will vary with time and depth after export. Overall these 234Th data provide a detailed view into particle flux
and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and
10’s to 100’s km at a resolution that is difficult to obtain with other methods.Funding for VERTIGO in the US was provided primarily by research grants from the US
National Science Foundation Programs in Chemical and Biological Oceanography with additional support by
the US Department of Energy (DAS). For TWT, support came from the Australian Cooperative Research
Centres program
Drivers of diatom production and the legacy of eutrophication in two river plume regions of the northern Gulf of Mexico
In the northern Gulf of Mexico (nGoM), the Louisiana Shelf (LS) and Mississippi Bight (MB) subregions are influenced by eutrophication to varying degrees. Despite recognition that dissolved silicon may regulate diatom productivity in the nGoM, there is only one published data set reporting biogenic silica (bSiO2) production rates for each subregion. We report that bSiO2 production rates on the LS and MB are high and appear to be controlled by different nutrients among seasons. Despite exceptional upper trophic level biomass regionally, which suggests significant primary production by diatoms (as in other systems), gross euphotic-zone integrated bSiO2 production rates are lower than major bSiO2 producing regions (e.g. upwelling systems). However, when normalizing to the depth of the euphotic zone, the bSiO2 production rates on the LS are like normalized rates in upwelling systems. We suggest local river-plume influenced hydrography concentrates diatom productivity within shallow euphotic zones, making production more accessible to higher trophic organisms. Comparison of rates between the LS and MB suggest that the fluvial nitrate within the LS stimulates bSiO2 production above that in the MB, which has a smaller watershed and is less eutrophic (relatively). Beyond understanding the factors controlling regional bSiO2 production, these data offer the most comprehensive Si-cycle baseline to date as the LS and MB will likely exchange freely in the mid to late century due to land subsidence of the Mississippi River delta and/or sea-level rise
Biogeochemical responses to late-winter storms in the Sargasso Sea, III—Estimates of export production using 234Th:238U disequilibria and sediment traps
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 56 (2009): 875-891, doi:10.1016/j.dsr.2009.01.008.Direct measurements of new production and carbon export in the subtropical
North Atlantic Ocean appear to be too low when compared to geochemical based
estimates. It has been hypothesized that episodic inputs of new nutrients into surface
water via the passage of mesoscale eddies or winter storms may resolve at least some of
this discrepancy. Here, we investigated particulate organic carbon (POC), particulate
organic nitrogen (PON), and biogenic silica (BSiO2) export using a combination of water
column 234Th:238U disequilibria and free-floating sediment traps during and immediately
following two weather systems encountered in February and March 2004. While these
storms resulted in a 2-4 fold increase in mixed layer NO3 inventories, total chlorophyll a
and an increase in diatom biomass, the systems was dominated by generally low
234Th:238U disequilibria, suggesting limited particle export. Several 234Th models were
tested, with only those including non-steady state and vertical upwelling processes able to
describe the observed 234Th activities. Although upwelling velocities were not measured
directly in this study, the 234Th model suggests reasonable rates of 2.2 to 3.7 m d-1.
Given the uncertainties associated with 234Th derived particle export rates and
sediment traps, both were used to provide a range in sinking particle fluxes from the
upper ocean during the study. 234Th particle fluxes were determined applying the more
commonly used steady state, 1-dimensional model with element/234Th ratios measured in
sediment traps. Export fluxes at 200 m ranged from 1.91 ± 0.20 to 4.92 ± 1.22 mmol C
m-2 d-1, 0.25 ± 0.08 to 0.54 ± 0.09 mmol N m-2 d-1, and 0.22 ± 0.04 to 0.50 ± 0.06 mmol
Si m-2 d-1. POC export efficiencies (Primary Production/Export) were not significantly
different from the annual average or from time periods without storms, although absolute
POC fluxes were elevated by 1-11%. This increase was not sufficient, however, to
resolve the discrepancy between our observations and geochemical based estimates of
particle export. Comparison of PON export rates with simultaneous measurements of
NO3
- uptake derived new production rates, suggested that only a fraction, < 35%, of new
production was exported as particles to deep waters during these events. Measured bSiO2
export rates were more than a factor of two higher (p < 0.01) than the annual average,
with storm events contributing as much as 50% of annual bSiO2 export in the Sargasso
Sea. Furthermore it appears that 65 - 95% (average 86 ± 14%) of the total POC export
measured in this study was due to diatoms.
Combined these results suggest that winter storms do not significantly increase
POC and PON export to depth. Rather, these storms may play a role in the export of
bSiO2 to deep waters. Given the slower remineralization rates of bSiO2 relative to POC
and PON, this transport may, over time, slowly decrease water column silicate
inventories, and further drive the Sargasso Sea towards increasing silica limitation.
These storm events may further affect the quality of the POC and PON exported given
the large association of this material with diatoms during these periods.This study was funded by the National Science Foundation (Chemical
Oceanography Grants OCE-0244612 and OCE-0241645)
Cobalt, manganese, and iron near the Hawaiian Islands : a potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1473-1490, doi:10.1016/j.dsr2.2008.02.010.The vertical distributions of cobalt, iron, and manganese in the water column were
studied during the E-Flux Program (E-Flux II and III), which focused on the
biogeochemistry of cold-core cyclonic eddies that form in the lee of the Hawaiian
Islands. During E-Flux II (January 2005) and E-Flux III (March 2005), 17 stations were
sampled for cobalt (n =147), all of which demonstrated nutrient-like depletion in surface
waters. During E-Flux III, two depth profiles collected from within a mesoscale coldcore
eddy, Cyclone Opal, revealed small distinct maxima in cobalt at ~100m depth and a
larger inventory of cobalt within the eddy. We hypothesize that this was due to a cobalt
concentrating effect within the eddy, where upwelled cobalt was subsequently associated
with sinking particulate organic carbon (POC) via biological activity and was released at
a depth coincident with nearly complete POC remineralization (Benitez-Nelson et al.
2007). There is also evidence for the formation of a correlation between cobalt and
soluble reactive phosphorus during E-Flux III relative to the E-Flux II cruise that we
suggest is due to increased productivity, implying a minimum threshold of primary
production below which cobalt-phosphate coupling does not occur. Dissolved iron was
measured in E-Flux II and found in somewhat elevated concentrations (~0.5nM) in
surface waters relative to the iron depleted waters of the surrounding Pacific (Fitzwater et
al. 1996), possibly due to island effects associated with the iron-rich volcanic soil from
the Hawaiian Islands and/or anthropogenic inputs. Distinct depth maxima in total
dissolved cobalt were observed at 400 to 600m depth, suggestive of the release of metals
from the shelf area of comparable depth that surrounds these islands.This research was supported by NSF Grants
OCE-0327225, OCE-0452883, OPP-0440840, the Office of Naval Research, the Center
for Environmental Bioinorganic Chemistry at Princeton, and the Center for Microbial
Oceanography and Education