Seasonal and spatial changes in carbon and nitrogen fluxes estimated using 234Th:238U disequilibria in the North Pacific tropical and subtropical gyre

Umhau, Blaire P.; Benítez-Nelson, Claudia R.; Close, Hilary G.; Hannides, Cecelia C. S.; Motta, Laura C.; Popp, Brian N.; Drazen, Jeffrey C.

doi:10.1016/j.marchem.2019.103705

Cited by 20 publications

(24 citation statements)

References 52 publications

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“…1A), where the contribution of small particles as a food source to deep zooplankton (fraction small particles, f SP ; see “Methods” section) was higher than expected. This suggests the dominance of smaller particles in the flux at this site, which agrees with Umhau et al (2019). Also, at 5°N a higher proportion of carbonates within the small particle pool was measured (Supporting Information Table S1), similar to previous results from the Equatorial Pacific during El Niño (Hernes et al 2001), probably due to the decline in abundance of diatoms and the dominance of nanoeucaryotes like coccolithophorids that are captured in the small size fraction.…”

Section: Resultssupporting

confidence: 92%

“…Total 234 Th was measured in samples of seawater collected at discrete depths using Niskin bottles as described in Umhau et al (2019). These samples were collected during the same period as particles and zooplankton at each site.…”

Section: Methodsmentioning

confidence: 99%

“…These samples were collected during the same period as particles and zooplankton at each site. The 234 Th flux (dpm m −2 d −1 ) at each depth was calculated from the 234 Th : 238 U disequilibrium and assuming a steady state (Umhau et al 2019). The 234 Th flux at 200 m was obtained by integrating the flux for the overlying depths.…”

Section: Methodsmentioning

confidence: 99%

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Deep zooplankton rely on small particles when particle fluxes are low

Romero-Romero

Ka'apu-Lyons

Umhau

et al. 2020

Limnol Oceanogr Letters

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Deep-sea communities rely on organic matter (OM) produced almost exclusively in surface waters. Pulses of fast sinking material are their main food source, but here we show that when the quantity of that material is low, small particles can sustain deep zooplankton communities. Small particles are often overlooked as a source of OM to deep food webs, yet they are found in higher concentrations than larger, faster sinking particles, and their relative contribution to the total pool is expected to increase in the future. Our results highlight the importance of small particles as a food source sustaining deep food webs, which may become more widespread given the predicted changes in overlying surface waters.

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Section: Resultssupporting

confidence: 92%

Section: Methodsmentioning

confidence: 99%

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Deep zooplankton rely on small particles when particle fluxes are low

Romero-Romero

Ka'apu-Lyons

Umhau

et al. 2020

Limnol Oceanogr Letters

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“…POC in the lower euphotic zone or upper pycnocline is abundant and serves as a food source for subsurface biota; studies such as Liu et al (2018) have discussed how variations in δ 13 C POC values in deep chlorophyll maxima can influence how δ 13 C values are interpreted in food web studies. The depths of the lower euphotic zone or upper pycnocline, which usually include deep chlorophyll maxima when present, can also be a location of net particle formation and export (Kemp et al, 2000;Benitez-Nelson et al, 2001;Umhau et al, 2019). Recognizing subsurface sources for exported material can be important to the interpretation of the sedimentary geochemical record (e.g., Luo et al, 2014;Hurley et al, 2018).…”

Section: Unique Geochemical Signatures Of the Lower Euphotic Zone Or mentioning

confidence: 99%

Open-Ocean Minima in δ13C Values of Particulate Organic Carbon in the Lower Euphotic Zone

Henderson

2020

Front. Mar. Sci.

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Extensive studies in the 1980s-1990s led to the characterization of latitudinal variations in sea surface δ 13 C values of particulate organic carbon (δ 13 C POC), and relationships were found with CO 2 concentrations, temperature, growth rates, and cell geometries. Surprisingly, no large-scale efforts have been made to describe variations in δ 13 C POC values over depth in the water column. Here we compile published examples demonstrating a widespread isotopic pattern in particulate organic carbon (POC) of the upper water column. In 51 vertical profiles, δ 13 C POC values in the lower euphotic zone on average are 1.4 lower than δ 13 C POC values in the upper euphotic zone of open ocean settings. In a majority of locations this vertical decrease in δ 13 C POC values is >2 and up to 5 , larger than the commonly recognized vertical δ 13 C variation in dissolved inorganic carbon over the same depths. We briefly review hypotheses and supporting evidence offered by previous studies of individual water columns: The observed patterns could result from vertical differences in photosynthetic growth rates or community composition, biochemical composition of organic matter due to degradation, isotopic disequilibrium within the dissolved inorganic carbon pool, particle dynamics, or seasonal vertical mixing. Coordinated isotopic, biological, and seawater chemistry data are sparse, and consistent drivers of this widespread isotopic pattern are currently elusive. Further work is needed to adequately characterize the environmental conditions coinciding with this pattern, to test its origins, and to determine if the magnitude of upper water column δ 13 C POC variations could be a useful marker of upper ocean carbon cycle dynamics.

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“…The short-lived radionuclide thorium-234 ( 234 Th, t 1/2 = 24.1 d) is widely used to estimate the magnitude of POC that escapes the upper ocean layers (e.g., the euphotic zone) (Waples et al, 2006). 234 Th is the decay product of uranium-238 ( 238 U, t 1/2 = 4.47 × 10 9 yr).…”

Section: The 234 Th Approachmentioning

confidence: 99%

Global database of ratios of particulate organic carbon to thorium-234 in the ocean: improving estimates of the biological carbon pump

Puigcorbé

Masqué

Moigne

2020

Earth Syst. Sci. Data

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Abstract. The ocean's biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g., marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 Gt C yr−1 exported below the euphotic zone or mixed-layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage of allowing a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to the POC∕234Th ratio in sinking particles. However, the factors controlling the regional, temporal, and depth variations of POC∕234Th ratios are poorly understood. We present a database of 9318 measurements of the POC∕234Th ratio in the ocean, from the surface down to >5500 m, sampled on three size fractions (∼>0.7 µm, ∼1–50 µm, ∼>50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived in the data repository PANGAEA® under https://doi.org/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018, and the data have been obtained from published papers and open datasets available online. Unpublished data have also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g., China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific, and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depth horizons and climate zones, the median POC∕234Th ratios have a wide range, from 0.6 to 18 µmol dpm−1.

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Seasonal and spatial changes in carbon and nitrogen fluxes estimated using 234Th:238U disequilibria in the North Pacific tropical and subtropical gyre

Cited by 20 publications

References 52 publications

Deep zooplankton rely on small particles when particle fluxes are low

Deep zooplankton rely on small particles when particle fluxes are low

Open-Ocean Minima in δ13C Values of Particulate Organic Carbon in the Lower Euphotic Zone

Global database of ratios of particulate organic carbon to thorium-234 in the ocean: improving estimates of the biological carbon pump

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