Export Flux Research Articles

Seasonal flux patterns and carbon transport from low-oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)

Abstract. Mesoscale eddies are abundant in the eastern tropical North Atlantic and act as oases for phytoplankton growth due to local enrichment of nutrients in otherwise oligotrophic waters. It is not clear whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies' contribution to bathypelagic particle flux are difficult to obtain. Rare observations of export flux associated with low-oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cabo Verde islands at the oligotrophic Cape Verde Ocean Observatory (CVOO; approx. 17∘35′ N, 24∘15′ W). The CVOO site is located in the preferred pathways of highly productive eddies that ultimately originate from the Mauritanian upwelling region. Between 2009 and 2016, we collected biogenic and lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water depths at the CVOO site. From concurrent hydrography and oxygen observations, we confirm earlier findings that highly productive eddies are characterized by colder and less saline waters and a low-oxygen signal as well. Overall, we observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux increases at 3 km depth during October–November when the eddies approached CVOO and distinct flux peaks during February–March, clearly exceeding low oligotrophic background fluxes during winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) showed a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass showed an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenic material had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear and turbulence, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with lithogenic material is assumed to originate from the interior of eddies or from mixed sources, both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing highly productive eddies at CVOO repeatedly release characteristic flux signals to the bathypelagic zone during winter–spring seasons that are far above the oligotrophic background fluxes and sequester higher organic carbon than during oligotrophic settings. However, the reasons for a lower carbon flux attenuation below eddies remain elusive.

Responses of Net Anthropogenic N Inputs and Export Fluxes in the Megacity of Chengdu, China

Anthropogenic N inputs have become progressively more problematic and have profoundly affected the water quality in megacities throughout China. Thus, to design and implement appropriate megalopolis watershed management, it is important to understand the relationship between N inputs and exports and to identify the N pollution sources. To that end, in this work, the net anthropogenic N inputs (NANI) in Chengdu City were estimated based on statistical data collected between 1970 and 2019. N input fluxes and pollution sources were estimated through sample collection and field measurements that were performed between 2017 and 2019, while nitrate (NO3−) was identified using stable isotope and Bayesian model (SIAR) analysis. The NANI was found to be affected primarily by livestock and poultry consumption of N rich feed. Moreover, the N export fluxes and runoff showed a high degree of correlation. Notably, NO3− fluxes exhibited a significant increase over the course of the study period, such that, by 2019, the total N fluxes (18,883.85 N kg/km2) exceeded the NANI (17,093.87 N kg/km2). The results indicate that although livestock and poultry farming were the original primary sources of NANI, their contributions declined on an annual basis. Moreover, with the emphasis placed on point source management in Chengdu City, domestic sewage discharge has been significantly reduced. Therefore, N retention in groundwater is thought to be the factor driving the N flux increase. These findings are pivotal to solving the N pollution problem in megacities like Chengdu (China).

Dissolved and suspended organic matter dynamics in the Cape Verde Frontal Zone (NW Africa)

The Cape Verde Frontal Zone (CVFZ) is a highly dynamic region located in the southern boundary of the Canary Current Eastern Boundary Upwelling Ecosystem. Due to the interaction of the Cape Verde Front with the Mauritanian coastal upwelling, the area features large vertical and horizontal export fluxes of organic matter. While the flux, composition and biogeochemical role of sinking organic matter have been thoroughly studied, much less attention has been paid to the dissolved (DOM) and suspended particulate (POM) organic matter fractions. Full-depth profiles of DOM and POM were recorded during an oceanographic cruise in the CVFZ, with four consecutive transects defining a box embracing the giant filament of Cape Blanc and the Cape Verde front. The distributions of DOM and POM and their C:N stoichiometric ratios in the epipelagic layer were strongly influenced by the position of the transects relative to the giant filament and the front. Geographical heterogeneity in POM and DOM distributions and elemental composition was also observed within each of the different water masses of contrasting origin present in the area (North and South Atlantic Central Water, Subpolar Mode Water, Mediterranean Water, Antarctic Intermediate Water, Labrador Sea Water and North East Atlantic Deep Water). These facts suggest that water masses properties are re-shaped by biogeochemical processes occurring within the CVFZ. Nevertheless, our analysis indicates that DOM and POM mineralisation represents only 8.1% of the inorganic carbon and 17.8% of the inorganic nitrogen produced by the local mineralisation of organic matter. Intense lateral export of POM and DOM out of the boundaries of the CVFZ is the likely reason behind these low contributions, which confirm the prominent role of sinking fluxes of organic matter for mineralisation processes in this region. The DOM distribution in the CVFZ interior is apparently affected by the dissolution of fast sinking particles.

210Po-210Pb Disequilibrium in the Western North Pacific Ocean: Particle Cycling and POC Export

Estimating the particulate organic carbon (POC) export flux from the upper ocean is fundamental for understanding the efficiency of the biological carbon pump driven by sinking particles in the oceans. The downward POC flux from the surface ocean based on 210Po-210Pb disequilibria in seawater samples from the western North Pacific Ocean (w-NPO) was measured in the early summer (May-June) of 2018. All the profiles showed a large 210Po deficiency relative to 210Pb in the euphotic zone (0–150 m), while this 210Po deficiency vanished below ∼500 m (with 210Po/210Pb ∼1 or > 1). A one-dimensional steady-state irreversible scavenging model was used to quantify the scavenging and removal fluxes of 210Po and 210Pb in the euphotic zone of the w-NPO. In the upper ocean (0–150 m), dissolved 210Po (D-Po) was scavenged into particles with a residence time of 0.6–5.5 year, and the 210Po export flux out of the euphotic zone was estimated as (0.33–3.49) × 104 dpm/m2/year, resulting in a wide range of particulate 210Po (P-Po) residence times (83–921 days). However, in the deep ocean (150–1,000 m), 210Po was transferred from the particulate phase to the dissolved phase. Using an integrated POC inventory and the P-Po residence times (Eppley model) in the w-NPO euphotic zone, the POC export fluxes (mmol C/m2/d) varied from 0.6 ± 0.2 to 8.8 ± 0.4. In comparison, applying the POC/210Po ratio of all (>0.45 μm) particles to 210Po export flux (Buesseler model), the obtained POC export fluxes (mmol C/m2/d) ranged from 0.7 ± 0.1 to 8.6 ± 0.8. Both Buesseler and Eppley methods showed enhanced POC export fluxes at stations near the continental shelf (i.e., Luzon Strait and the Oyashio-Kuroshio mixing region). The Eppley model-based 210Po-derived POC fluxes agreed well with the Buesseler model-based fluxes, indicating that both models are suitable for assessing POC fluxes in the w-NPO. The POC export efficiency was < 15%, suggesting a moderate biological carbon pump efficiency in the w-NPO. These low export efficiencies may be associated with the dominance of smaller particles and the processes of degradation and subsequent remineralization of these small particles in the euphotic zone of oligotrophic regions in the w-NPO.

δ53Cr values of catchment runoff exhibit seasonality regardless of bedrock Cr concentrations: Role of periodically changing chromium export fluxes

AbstractOne pre‐requisite for the construction of a global chromium isotope mass balance is detailed understanding of Cr isotope systematics in the critical zone where redox‐processes can modify the isotope signature of geogenic Cr input into the hydrosphere. A Cr isotope inventory of bedrock, soil, and runoff was performed in a Central European headwater catchment underlain by amphibolite, situated in the vicinity of two previously studied catchments underlain by different bedrock types (serpentinite and leucogranite). Fresh bedrock in the amphibolite catchment NAZ contained ~300 mg/kg Cr, serpentinite at PLB contained ~800 mg/kg Cr, and leucogranite at LYS contained ~2 mg/kg Cr. Monthly hydrochemical monitoring at all three sites revealed higher Cr(VI) export fluxes in winter than in summer. NAZ was characterized by a distinct seasonality in the δ53Cr values, with minima during winter/spring snowmelts (−0.35‰) and maxima during dry summers (0.40‰). Similar seasonality in δ53Cr values had been reported from PLB and LYS. Bedrock at all three sites had similar Cr isotope composition close to −0.10‰, a value indistinguishable from the δ53Cr value of bulk silicate Earth (BSE). Positive mean δ53Cr value of NAZ runoff indicated Cr‐isotope fractionations during weathering of geogenic Cr(III), combined with adsorption of the resulting Cr(VI) on soil particles during pedogenesis. However, the mass‐weighted mean δ53Cr of NAZ runoff was lower (−0.08‰), indistinguishable from the Cr isotope signature of bedrock. The same pattern of lower mass‐weighted mean δ53Cr values of runoff, compared to arithmetic mean δ53Cr values of runoff, were observed also at PLB and LYS. We suggest that elevated Cr runoff fluxes in winter remove some of the residual isotopically light Cr that accumulated in the soil during summer. Seasonality in runoff δ53Cr values appears to be a relatively widespread phenomenon, de‐coupled from Cr availability for chemical weathering.

Simulations With the Marine Biogeochemistry Library (MARBL)

AbstractThe Marine Biogeochemistry Library (MARBL) is a prognostic ocean biogeochemistry model that simulates marine ecosystem dynamics and the coupled cycles of carbon, nitrogen, phosphorus, iron, silicon, and oxygen. MARBL is a component of the Community Earth System Model (CESM); it supports flexible ecosystem configuration of multiple phytoplankton and zooplankton functional types; it is also portable, designed to interface with multiple ocean circulation models. Here, we present scientific documentation of MARBL, describe its configuration in CESM2 experiments included in the Coupled Model Intercomparison Project version 6 (CMIP6), and evaluate its performance against a number of observational data sets. The model simulates present‐day air‐sea CO2 flux and many aspects of the carbon cycle in good agreement with observations. However, the simulated integrated uptake of anthropogenic CO2 is weak, which we link to poor thermocline ventilation, a feature evident in simulated chlorofluorocarbon distributions. This also contributes to larger‐than‐observed oxygen minimum zones. Moreover, radiocarbon distributions show that the simulated circulation in the deep North Pacific is extremely sluggish, yielding extensive oxygen depletion and nutrient trapping at depth. Surface macronutrient biases are generally positive at low latitudes and negative at high latitudes. CESM2 simulates globally integrated net primary production (NPP) of 48 Pg C yr−1 and particulate export flux at 100 m of 7.1 Pg C yr−1. The impacts of climate change include an increase in globally integrated NPP, but substantial declines in the North Atlantic. Particulate export is projected to decline globally, attributable to decreasing export efficiency associated with changes in phytoplankton community composition.

Exploring the impact of climate change on the global distribution of non-spinose planktonic foraminifera using a trait-based ecosystem model.

Planktonic foraminifera are one of the primary calcifiers in the modern ocean, contributing 23%-56% of total global pelagic carbonate production. However, a mechanistic understanding of how physiology and environmental conditions control their abundance and distribution is lacking, hindering the projection of the impact of future climate change. This understanding is important, not only for ecosystem dynamics, but also for marine carbon cycling because of foraminifera's key role in carbonate production. Here we present and apply a global trait-based ecosystem model of non-spinose planktonic foraminifera ('ForamEcoGEnIE') to assess their ecology and global distribution under future climate change. ForamEcoGEnIE considers the traits of calcium carbonate production, shell size, and foraging. It captures the main characteristic of biogeographical patterns of non-spinose species - with maximum biomass concentrations found in mid- to high-latitude waters and upwelling areas. The model also reproduces the magnitude of global carbonate production relatively well, although the foraminifera standing stock is systematically overestimated. In response to future scenarios of rising atmospheric CO2 (RCP6 and RCP8.5), on a regional scale, the modelled foraminifera biomass and export flux increases in the subpolar regions of the North Atlantic and the Southern Ocean while it decreases everywhere else. In the absence of adaptation, the biomass decline in the low-latitude South Pacific suggests extirpation. The model projects a global average loss in non-spinose foraminifera biomass between 8% (RCP6) and 11% (RCP8.5) by 2050 and between 14% and 18% by 2100 as a response to ocean warming and associated changes in primary production and ecological dynamics. Global calcium carbonate flux associated with non-spinose foraminifera declines by 13%-18% by 2100. That decline can slow down the ocean carbonate pump and create short-term positive feedback on rising atmospheric pCO2 .

Hydrothermal plumes as hotspots for deep-ocean heterotrophic microbial biomass production

Carbon budgets of hydrothermal plumes result from the balance between carbon sinks through plume chemoautotrophic processes and carbon release via microbial respiration. However, the lack of comprehensive analysis of the metabolic processes and biomass production rates hinders an accurate estimate of their contribution to the deep ocean carbon cycle. Here, we use a biogeochemical model to estimate the autotrophic and heterotrophic production rates of microbial communities in hydrothermal plumes and validate it with in situ data. We show how substrate limitation might prevent net chemolithoautotrophic production in hydrothermal plumes. Elevated prokaryotic heterotrophic production rates (up to 0.9 gCm−2y−1) compared to the surrounding seawater could lead to 0.05 GtCy−1 of C-biomass produced through chemoorganotrophy within hydrothermal plumes, similar to the Particulate Organic Carbon (POC) export fluxes reported in the deep ocean. We conclude that hydrothermal plumes must be accounted for as significant deep sources of POC in ocean carbon budgets.

Linkages Between East China Sea Deep‐Sea Oxygenation and Variability in the East Asian Summer Monsoon and Kuroshio Current Over the Last 400,000 years

AbstractThe East China Sea (ECS) seasonally receives a high organic input due to the terrestrial organic matter influx, which is controlled by the East Asian Summer Monsoon (EASM), and the increased productivity driven by upwelling of the subsurface Kuroshio Current (KC). Changes in benthic foraminiferal assemblage composition in combination with paleoceanographic proxy data (CaCO3 (%), TOC (%), δ13Cpf, and δ18Obf) are used to reconstruct bottom water oxygenation and organic export flux variability over the last 400 kyr in the ECS. Multivariate analyses of benthic foraminiferal census data identified six biofacies characteristic of varying environmental conditions. These results suggest that enhanced EASM precipitation and KC upwelling directly influenced organic export flux and bottom water oxygen content in the ECS. The ECS bottom water was suboxic during Marine Isotope Stage (MIS) 11 to 8; suboxic to dysoxic between MIS 7 and 6, strongly dysoxic between mid‐MIS 5 and 4, and exhibited high variability between MIS 3 and 1. Spectral analysis of relative abundances of representative genera Quinqueloculina (oxic), Bulimina (suboxic), and Globobulimina (dysoxic) reveals a robust 23 kyr signal, which we attribute to precessionally‐paced changes in surface productivity and bottom water oxygenation related to KC variability over the past 400 kyr.

Site-specific interactions enhanced dissolution of natural aragonite (110) surfaces in succinic acid (SUC) solutions: Implications for the oceanic aragonite dissolution fluxes

Aragonite, one of the most common biological calcium carbonate minerals, is widespread in marine plankton and neritic sediments (e.g., accounting for 89% of pelagic calcification of CaCO3 in the surface ocean). Its dissolution directly affects the export fluxes of CaCO3 in seawater. The aragonite dissolution in the ocean correlates with the increase of partial pressure of carbon dioxide and is pertinent to acidic biomolecules. However, aragonite dissolution in seawater with acidic biomolecules has been overlooked, and the interaction mechanism is unclear. In-situ atomic force microscopy (AFM) was employed to observe the dissolution features of aragonite (110) growth surfaces in succinic acid (SUC, HOOC-CH2-CH2-COOH) solutions. The results demonstrate that (1) both the morphologies and spreading rates of the etch pits formed on aragonite (110) surfaces are altered by the interactions between SUC molecules and the surfaces; (2) the [11‾1] and [1‾11] steps of the etch pits on aragonite (110) surfaces in SUC solutions are kinetically controlled; (3) dissolution rates of aragonite (110) surfaces are proportional to SUC concentrations, which is attributed to the strong complexation between SUC molecules and surface-bounded Ca atoms (≡Ca+); (4) etch pits morphologies of aragonite (110) and calcite (10.4) surfaces are different in SUC solutions, and the spreading rates of the etch pits of the former are one to two orders of magnitude lower than those of the latter. Furthermore, aragonite is found to be more sensitive to SUC molecules than calcite, suggesting that dissolving aragonitic materials could be more easily affected by circumambient biomolecules than dissolving calcitic materials. In seawater that contains organic molecules, the dissolution fluxes of aragonite could be much slower than those of calcite. These findings not only reveal the site-specific interactions between SUC molecules and the ≡Ca+ on aragonite (110) surfaces but also emphasize its implication for the oceanic aragonite dissolution fluxes.

Drawdown of Atmospheric pCO2 Via Variable Particle Flux Stoichiometry in the Ocean Twilight Zone

AbstractThe strength of the biological soft tissue pump in the ocean critically depends on how much organic carbon is produced via photosynthesis and how efficiently the carbon is transferred to the ocean interior. For a given amount of limiting nutrient, phosphate, soft tissue pump would be strengthened if the carbon (C) to phosphorus (P) ratio of sinking organic matter increases as the remineralization length scale of C increases. Here, we present a new data compilation of particle flux stoichiometry and show that C:P of sinking particulate organic matter (POM) in the ocean twilight zone on average is likely to be higher than the C:P ratio of surface suspended POM. We further demonstrate using a physics‐biology coupled global ocean model combined with a theory from first principles that an increase in C:P export flux ratio in the ocean's twilight zone can lead to a considerable drawdown of atmospheric pCO2.

Utilization of Soot and 210 Po-210 Pb Disequilibria to Constrain Particulate Organic Carbon Fluxes in the Northeastern South China Sea

Black carbon (BC) is believed to be refractory and thus affects the timescale of organic carbon conversion into CO2 and the magnitude of the sink of CO2. However, the fate of BC in the oceans remains poorly understood. Here, 210Po and 210Pb were measured to examine the export of soot in the northeastern South China Sea (SCS). Concentrations of soot decreased from 0.141 ± 0.021 μmol-C L–1 (mean ± SD) in the mixed layer (0–30 m) to 0.087 μmol-C L–1 at the euphotic base (150 m) due to potential photodegradation within the euphotic zone. In the twilight zone, however, the soot showed an increasing pattern along with the total particulate matter and total particulate organic carbon (POC) contents, corresponding to additions from the shelf/slope sediment resuspension through lateral transport. Using the deficits of 210Po, the export flux of soot from the euphotic zone was calculated to be 0.172 ± 0.016 mmol-C m–2 d–1 and increased with depth. Assuming that the soot is entirely refractory below the euphotic zone, the sediment-derived soot fluxes were estimated based on the increase in soot fluxes relative to the base of the euphotic zone, with values varying from 0.149 ± 0.030 to 0.96 ± 0.10 μmol-C L–1. This indicates that sediment resuspension is an important source of soot to the ocean interior in the SCS. Coupling the sediment-derived soot and 210Po-derived POC fluxes gave rise to a Martin Curve-like flux attenuation of local euphotic zone-derived POC in the twilight zone with b value of 0.70 ± 0.01. These results suggest that soot could be useful for constraining in situ POC fluxes and their transport.

A novel animal-borne miniature echosounder to observe the distribution and migration patterns of intermediate trophic levels in the Southern Ocean

Despite expanding in-situ observations of marine ecosystems by new-generation sensors, information about intermediate trophic levels remains sparse. Indeed, mid-trophic levels, while encompassing a broad range of zooplankton and micronekton organisms that represent a key component of marine ecosystems and sustain large and diverse communities of marine predators, are challenging to sample and identify. In this study, we examined whether an animal-borne miniature active echosounder can provide information on the distribution and movements of mid-trophic level organisms. If so, such a sonar tag, harnessing the persistent diving behaviour of far-ranging marine mammals, could greatly increase the density of data on this under-studied biome. High-frequency (1.5 MHz) sonar tags were deployed simultaneously with oceanographic tags on two southern elephant seals (Mirounga leonina), at the Kerguelen Islands and Valdés Peninsula (Argentina), and recorded acoustic backscatter while the seals foraged respectively in the Indian and the Atlantic sectors of the Southern Ocean. The backscatter varied widely over time and space, and the seals attempted to capture only a small fraction of the insonified targets. Diel vertical migration patterns were clearly identifiable in the data, reinforcing our confidence in the ability of the sonar tags to detect living mid-trophic organisms along with possibly sinking biological detritus. Moreover, CTD tags attached to the same animals indicated how the abundance, size distribution, and diel migration behaviour of acoustic targets varied with water bodies. These preliminary results demonstrate the potential for animal-borne sonars to provide detailed in-situ information. Further validation effort will make it a valuable tool to refine the estimation of carbon export fluxes as well as for assessing the variation of mid-trophic level biomass according to oceanographic domains and seasons.

Decreasing Biological Production and Carbon Export Due to the Barrier Layer: A Case Study in the Bay of Bengal

A freshwater-induced barrier layer (BL) is a common physical phenomenon both in coastal waters and the open ocean. To examine the effects of BL on the biological production and the associated carbon export, a physical-biogeochemical survey was conducted in the Bay of Bengal. Severe depletions of surface phosphorus and the deepening of the nutricline were observed at the BL-affected stations due to the vertical mixing prohibition. The lowered surface chlorophyll a (Chl a) and squeezed deep Chl a maximum (DCM) layer also resulted in the ~18% lowered vertically integrated Chl a at the said stations. The composition of the net-sampled zooplankton was altered, and the abundance decreased by half at the BL-affected station (29.68 ind. m−3) compared with the unaffected station (55.52 ind. m−3). Such reductions in major zooplankton groups were confirmed by a video plankton recorder (VPR). The VPR observation indicated that there was a lower (by one-half) abundance of detritus at the BL-affected station, while the much lower carbon export flux rates were estimated to be at the BL-affected station (0.31 mg C m−2 d−1) rather than the unaffected station (0.77 mg C m−2 d−1). An idealized one-dimensional nutrient-phytoplankton-detritus model identified that the existence of BL can lead to decreased surface nutrients and phytoplankton concentrations, squeezed DCM layers, and lower detritus abundances. Finally, this study indicated that BL layers inhibit biological production and reduce carbon export, thus playing an important role in the ocean biogeochemical cycles.

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Apparent oxygen utilization rates based on tritium-helium dating in the South China Sea: Implications for export production

We present tritium (3H) and helium isotope (δ3He) data from the South China Sea (SCS), to estimate apparent oxygen utilization rates (AOURs). The observed δ3He values are close to the theoretical solubility equilibrium value of -1.7% in the upper mixed layer, followed by an increase with depth down to ~1500 m. Below 1500 m depth, δ3He is homogenously distributed with a value of 20.9% ± 1.0%. The distribution of δ3He reveals that a significant fraction of 3He throughout the water column over the entire SCS basin is allochthonous, derived from mantle sources originated from the Pacific Ocean. By using the salinity-normalized “potential” alkalinity (NPA) as a conservative mixing tracer, 3He produced by tritium decay (tritiogenic) was separated from mantle 3He, and the apparent 3H–3He ages of the SCS water masses were subsequently calculated to be 11 ± 5 years at 100 m depth and 50 ± 4 years at 1000 m. Together with the observed dissolved oxygen concentrations, we estimated the mean AOURs and obtained values of 4.15 ± 0.27 μmol kg−1·yr−1 for the depth range of 100–500 m, and 0.81 ± 0.23 μmol kg−1·yr−1 for that between 800 and 1000 m depth. The depth-integrated AOURs between 100 and 1000 m depth, yield a spatially and temporally averaged export flux of organic carbon of 1.96 ± 0.16 mol-C·m−2·yr−1, comparable with previous estimates based on either 238U/234Th disequilibrium or nutrient and oxygen mass balance calculations in the SCS.

Impact of a warm anomaly in the Pacific Arctic region derived from time-series export fluxes.

Unusually warm conditions recently observed in the Pacific Arctic region included a dramatic loss of sea ice cover and an enhanced inflow of warmer Pacific-derived waters. Moored sediment traps deployed at three biological hotspots of the Distributed Biological Observatory (DBO) during this anomalously warm period collected sinking particles nearly continuously from June 2017 to July 2019 in the northern Bering Sea (DBO2) and in the southern Chukchi Sea (DBO3), and from August 2018 to July 2019 in the northern Chukchi Sea (DBO4). Fluxes of living algal cells, chlorophyll a (chl a), total particulate matter (TPM), particulate organic carbon (POC), and zooplankton fecal pellets, along with zooplankton and meroplankton collected in the traps, were used to evaluate spatial and temporal variations in the development and composition of the phytoplankton and zooplankton communities in relation to sea ice cover and water temperature. The unprecedented sea ice loss of 2018 in the northern Bering Sea led to the export of a large bloom dominated by the exclusively pelagic diatoms Chaetoceros spp. at DBO2. Despite this intense bloom, early sea ice breakup resulted in shorter periods of enhanced chl a and diatom fluxes at all DBO sites, suggesting a weaker biological pump under reduced ice cover in the Pacific Arctic region, while the coincident increase or decrease in TPM and POC fluxes likely reflected variations in resuspension events. Meanwhile, the highest transport of warm Pacific waters during 2017–2018 led to a dominance of the small copepods Pseudocalanus at all sites. Whereas the export of ice-associated diatoms during 2019 suggested a return to more typical conditions in the northern Bering Sea, the impact on copepods persisted under the continuously enhanced transport of warm Pacific waters. Regardless, the biological pump remained strong on the shallow Pacific Arctic shelves.

WAP-1D-VAR v1.0: development and evaluation of a one-dimensional variational data assimilation model for the marine ecosystem along the West Antarctic Peninsula

Abstract. The West Antarctic Peninsula (WAP) is a rapidly warming region, with substantial ecological and biogeochemical responses to the observed change and variability for the past decades, revealed by multi-decadal observations from the Palmer Antarctica Long-Term Ecological Research (LTER) program. The wealth of these long-term observations provides an important resource for ecosystem modeling, but there has been a lack of focus on the development of numerical models that simulate time-evolving plankton dynamics over the austral growth season along the coastal WAP. Here, we introduce a one-dimensional variational data assimilation planktonic ecosystem model (i.e., the WAP-1D-VAR v1.0 model) equipped with a model parameter optimization scheme. We first demonstrate the modified and newly added model schemes to the pre-existing food web and biogeochemical components of the other ecosystem models that WAP-1D-VAR model was adapted from, including diagnostic sea-ice forcing and trophic interactions specific to the WAP region. We then present the results from model experiments where we assimilate 11 different data types from an example Palmer LTER growth season (October 2002–March 2003) directly related to corresponding model state variables and flows between these variables. The iterative data assimilation procedure reduces the misfits between observations and model results by 58 %, compared to before optimization, via an optimized set of 12 parameters out of a total of 72 free parameters. The optimized model results capture key WAP ecological features, such as blooms during seasonal sea-ice retreat, the lack of macronutrient limitation, and modeled variables and flows comparable to other studies in the WAP region, as well as several important ecosystem metrics. One exception is that the model slightly underestimates particle export flux, for which we discuss potential underlying reasons. The data assimilation scheme of the WAP-1D-VAR model enables the available observational data to constrain previously poorly understood processes, including the partitioning of primary production by different phytoplankton groups, the optimal chlorophyll-to-carbon ratio of the WAP phytoplankton community, and the partitioning of dissolved organic carbon pools with different lability. The WAP-1D-VAR model can be successfully employed to link the snapshots collected by the available data sets together to explain and understand the observed dynamics along the coastal WAP.

Oxygen Isotope Offsets in Deep-Water Benthic Foraminifera

ABSTRACT Despite the extensive use of the benthic foraminiferal oxygen isotope composition (δ18O) as a proxy for paleoclimatic reconstructions, uncertainties remain regarding the consistency of interspecies offsets and the environmental factors controlling 18O fractionation. We investigated δ18O offsets of some frequently used Uvigerina, Bulimina, and Cibicidoides species in core top samples from different hydrographic and sedimentary regimes in the South China Sea, Makassar Strait, and Timor Strait/Eastern Indian Ocean. The δ18O values of the epifaunal taxa Cibicidoides mundulus and Cibicidoides wuellerstorfi showed no significant offset in all investigated regions, whereas shallow infaunal Cibicidoides species exhibited higher variability and were less reliable. We found no offsets between species of Uvigerina and Bulimina and assume that these genera can be measured together and/or substituted. Our results show that epifaunal taxa are close to equilibrium with ambient seawater and thus provide more reliable records of past ice volume and/or bottom water temperature variations than infaunal taxa. Offsets among equilibrium calcite, epifaunal taxa, and infaunal taxa are not constant “vital effects” but are influenced by changing gradients in bottom to pore water pH and carbonate ion concentrations that depend on deep-water ventilation and export flux of particulate carbonate and organic carbon. Offsets between epifaunal and infaunal taxa varied between 0.58 and 0.73‰, depending on regional bottom and pore water conditions. Our findings highlight the importance of regional and temporal variations in organic carbon flux/degradation and dissolution of calcite that may lead to slight under- or overestimates of the amplitude of δ18O fluctuations, especially during times of rapidly changing calcite-saturation of bottom and pore water.

Linking diatom-diazotroph symbioses to nitrogen cycle perturbations and deep-water anoxia: Insights from Mediterranean sapropel events

Elevated organic matter (OM) export flux promotes marine anoxia, thus increasing carbon sequestration efficiency and decreasing atmospheric carbon dioxide levels. However, the mechanisms that trigger and sustain anoxic events—particularly those associated with nutrient-poor, oligotrophic surface waters—remain poorly constrained. Mediterranean Sea sapropels are well-preserved sediments deposited during episodic anoxic events throughout the Plio-Pleistocene; as such, they may provide unique insight into the biogeochemical and ecological drivers of—and responses to—marine anoxia. Using biomarker distributions, we demonstrate that anaerobic ammonium oxidizing (anammox) bacteria and diazotrophic endosymbionts of mat- and/or raft-forming diatoms were both abundant during sapropel events, particularly in the Ionian and Libyan seas. In these sapropels, the carbon isotope compositions of anammox biomarkers directly capture progressive 13C-depletion in deep-water dissolved inorganic carbon, indicating sustained carbon sequestration. To explain these observations, we propose a reinforcing feedback whereby initial nutrient and/or circulation perturbations promote fixed nitrogen loss via intensified anammox and heterotrophic denitrification, which in turn favors proliferation of rapidly sinking diatom-diazotroph symbiotic consortia, increases OM burial flux, and sustains anoxia. This mechanism resolves the long-standing conundrum that small and buoyant diazotrophs are apparently associated with high OM export during periods of marine anoxia and oligotrophy.