Ocean acidification is projected to lower the Ωar of reefal waters by 0.3–0.4 units by the end of century, making it more difficult for calcifying organisms to secrete calcium carbonate while at the ...same time making the environment more favorable for abiotic and biotic dissolution of the reefal framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reefal carbonate production. This study represents the first broad‐scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009–2010 across seven onshore‐offshore transects spanning the upper, middle, and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m−2 d−1, respectively, and negative during the fall/winter at −33 ± 6 and −7 ± 2 mmol m−2 d−1. The most significant finding of the study was that the northernmost reef is already net erosional (−1.1 ± 0.4 kg CaCO3 m−2 yr−1) and midreefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m−2 yr−1) but erosional during the fall and winter. Only the two southernmost reefs were net depositional year‐round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.
Key Points
Seasonal reversal in the sign of NCP drives positive feedback on NCC in the summer and negative feedback in the winter
Seasonal net dissolution in waters supersaturated with respect to aragonite
Net loss of reefal substrate due to ocean acidification is already occurring
The distributions of iodate and iodide were measured along the GEOTRACES GP15 meridional transect at 152°W from the shelf of Alaska to Papeete, Tahiti. The transect included oxygenated waters near ...the shelf of Alaska, the full water column in the central basin in the North Pacific Basin, the upper water column spanning across seasonally mixed regimes in the north, oligotrophic regimes in the central gyre, and the equatorial upwelling. Iodide concentrations are highest in the permanently stratified tropical mixed layers, which reflect accumulation due to light‐dependent biological processes, and decline rapidly below the euphotic zone. Vertical mixing coefficients (Kz), derived from complementary 7Be data, enabled iodide oxidation rates to be estimated at two stations. Iodide half‐lives of 3–4 years show the importance of seasonal mixing processes in explaining north‐south differences in the transect, and also contribute to the decrease in iodide concentrations with depth below the mixed layer. These estimated half‐lives are consistent with a recent global iodine model. No evidence was found for significant inputs of iodine from the Alaskan continental margin, but there is a significant enrichment of iodide in bottom waters overlying deep sea sediments from the interior of the basin.
Plain Language Summary
Iodine is an important element in the oceans' biology and chemistry. The two principal forms are iodate and iodide, which were measured on a surface to seafloor survey from Alaska to Tahiti in 2018 as part of the international GEOTRACES program. Iodate, the stable form in the presence of oxygen, was predominant throughout the transect, but iodide was present at the boundaries, including the Alaskan margin, the abyssal plain, and surface waters. A "hot zone” of high iodide comprising up to 50% of the total iodine concentration was detected in tropical surface waters centered on the equator. It reflects accumulation from biological reduction of iodate by phytoplankton coupled with re‐oxidation of iodide by a non‐light‐dependent process. Iodate reduction probably occurs throughout the transect, but winter mixing of surface waters during storms probably smears out this feature in northern waters. This suggests that the turnover between iodide and iodate is longer than seasonal timescales. Indeed, estimates of iodide half‐life in the range of 3–4 years were calculated from complimentary shipboard measurements of a short‐lived radioisotope, beryllium‐7. High iodide in equatorial surface waters is important for atmospheric chemistry as it has a big impact on the global ozone budget.
Key Points
Iodate is the predominant form of iodine in deep waters, but iodide strongly accumulates in the tropical mixed layer
Iodide oxidation rates were estimated using complimentary Be‐7 data, indicating a half‐life in the lower euphotic zone of 3–4 years
Sediments on the abyssal plain are a modest source of iodide to bottom waters
The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling (PCU) region. In this work, the flux of nitrate into the euphotic zone is ...derived for stations within the PCU using a previously developed method whereby dilution of the water column 7Be inventory by upwelled 7Be‐free water provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, 7Be profiles are used to constrain vertical diffusivity within the upper thermocline. These transport terms are applied to nitrate profiles to estimate net community production between 79°W and 104°W along the EPZT, which includes the zone of active upwelling to the edge of the oligotrophic gyre. With a simple, one‐dimensional model, the calculated upwelling rates were inversely related to mixed layer temperature and ranged from 0 to 3.0 m/d. Results using a depth‐dependent upwelling rate with a component of horizontal advection are also described. Vertical diffusivities near the base of the euphotic zone were in the range 1.7–4.5 × 10−4 m2/s. These values are compared to those generated by analysis of temperature profiles. Net community production averaged 15 mmol C/m2/d for stations between 84°W and 104°W and was 134 mmol C/m2/d for the furthest inshore station at 79°W which displayed the lowest SST and greatest rate of upwelling.
Key Points
The 2013 U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT) traversed the highly productive Peruvian coastal upwelling region (PCU)
The cosmogenic isotope 7Be was used to estimate upwelling rates and vertical diffusivity in the upper thermocline of the PCU
The 7Be‐derived vertical transport terms were applied to nitrate profiles to derive net community production
Metabolic activity in the water column below the euphotic zone is ultimately fuelled by the vertical flux of organic material from the surface. Over time, the deep ocean is presumably at steady ...state, with sources and sinks balanced. But recently compiled global budgets and intensive local field studies suggest that estimates of metabolic activity in the dark ocean exceed the influx of organic substrates. This imbalance indicates either the existence of unaccounted sources of organic carbon or that metabolic activity in the dark ocean is being over-estimated. Budgets of organic carbon flux and metabolic activity in the dark ocean have uncertainties associated with environmental variability, measurement capabilities, conversion parameters, and processes that are not well sampled. We present these issues and quantify associated uncertainties where possible, using a Monte Carlo analysis of a published data set to determine the probability that the imbalance can be explained purely by uncertainties in measurements and conversion factors. A sensitivity analysis demonstrates that the bacterial growth efficiencies and assumed cell carbon contents have the greatest effects on the magnitude of the carbon imbalance. Two poorly quantified sources, lateral advection of particles and a population of slowly settling particles, are discussed as providing a means of closing regional carbon budgets. Finally, we make recommendations concerning future research directions to reduce important uncertainties and allow a better determination of the magnitude and causes of the unbalanced carbon budgets.
Naturally formed forest patches known as tree islands are found within lower-statured wetland matrices throughout the world, where they contrast sharply with the surrounding vegetation. In some ...coastal wetlands they are embedded in former freshwater marshes that are currently exposed to saltwater intrusion and mangrove encroachment associated with accelerating sea-level rise. In this study we resurveyed tree composition and determined environmental conditions in tree islands of the coastal Florida Everglades that had been examined two decades earlier. We asked whether tree islands in this coastal transition zone were differentiated geomorphologically as well as compositionally, and whether favorable geomorphology enabled coastal forest type(s) to maintain their compositional integrity against rising seas. Patterns of variation in geomorphology and soils among forest types were evident, but were dwarfed by differences between forest and adjacent wetlands. Tree island surfaces were elevated by 12–44 cm, and
210
Pb analyses indicated that their current rates of vertical accretion were more rapid than those of surrounding ecosystems. Tree island soils were deeper and more phosphorus-rich than in the adjoining matrix. Salinity decreased interiorward in both tree island and marsh, but porewater was fresher in forest than marsh in Mixed Swamp Forest, midway along the coastal gradient where tropical hardwoods were most abundant. Little decrease in the abundance of tropical hardwood species nor increase in halophytes was observed during the study period. Our data suggest that geomorphological differences between organic tree island and marl marsh, perhaps driven by groundwater upwelling through more transmissive tree island soils, contributed to the forests’ compositional stability, though this stasis may be short-lived despite management efforts.
Aerosol deposition is an important pathway for delivering trace elements, including those of anthropogenic origin, into the Arctic. Assessment of this process is difficult in the harsh Arctic ...environment, and limited field studies have forced a reliance on poorly constrained models. Here we use the cosmic ray produced radioisotope, 7Be, to trace the atmospheric deposition of elements within the Arctic water/ice/snow system, and link aerosol concentrations to flux. Seawater, ice, snow, melt pond, and aerosol samples were collected during late summer 2011 as part of the RV Polarstern's ARK-XXVI/3 campaign. From the measured 7Be inventories we determined an average 7Be flux of 109dpm/m2/d, which is consistent with results from previous studies in the region. Snow, ice and melt ponds represent significant reservoirs of 7Be, and the relative 7Be inventory in ice increased through late August, as melt pond inventories decreased with onset of freezing. The total (water/ice/snow system) inventory was relatively constant across our transect, but mixed layer inventories increased towards lower latitudes as ice-free, open water was approached. The latter gradient drives transport of 7Be, and presumably other atmospherically-derived species, towards the ice-covered ocean mixed layer. This is modeled by advective transport along the Transpolar Drift. The average 7Be aerosol concentration was 0.0182dpm/m3. None of the lithogenic aerosol elements showed any significant enrichment above crustal composition, while the pollution-derived elements (Cr, Ni, Cu, Zn, Cd, Sb, Pb) showed varying degrees of enrichment relative to crustal values. Historical aerosol 7Be data was used to derive a seasonal cycle in the 7Be inventory that was calibrated to the inventory measured in this study, using an effective bulk (wet plus dry) deposition velocity of 1350m/day. This deposition velocity was then used to estimate the seasonal atmospheric flux of aerosol trace elements.
•7Be was used to trace atmospheric deposition of elements within the Arctic Ocean.•An average 7Be flux of 109dpm/m2/d was determined.•This flux was partitioned between the ocean, ice, snow and melt ponds.•An effective bulk deposition velocity of 1350m/day was derived from the 7Be inventory.•This deposition velocity was used to estimate the atmospheric flux of trace elements.
The rate of oxygen utilization beneath the sunlit surface ocean provides a measure of the export rate of biologically produced carbon to the deep sea, and its variation with depth suggests where ...remineralization of that carbon occurs. The latter consideration is relevant to the efficiency of carbon sequestration in deep water. However, accurate characterization of this process, particularly within 200 m of the euphotic zone where carbon utilization is most intense, has been difficult owing to limitations of techniques applied to these shallow depths. Here, a novel approach utilizing the cosmogenic isotope 7Be indicates that at a site in the subtropical North Atlantic, 65% of sinking carbon is remineralized within 200 m of the ocean's surface and is thus readily available for return to the atmosphere. The corresponding oxygen utilization rates are greater than would be suggested by the attenuation with depth of organic matter measured by shallow sediment traps.
Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of ...physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.
Melt ponds are a prominent feature of Arctic sea ice during the summer and play a role in the complex interface between the atmosphere, cryosphere and surface ocean. During melt pond formation and ...development, micronutrient and contaminant trace elements (TEs) from seasonally accumulated atmospheric deposition are mixed with entrained sedimentary and marine-derived material before being released to the surface ocean during sea ice melting. Here we present particulate and size-fractionated dissolved (truly soluble and colloidal) TE data from five melt ponds sampled in late summer 2015, during the US Arctic GEOTRACES (GN01) cruise. Analyses of salinity, δ18O, and 7Be indicate variable contributions to the melt ponds from snowmelt, melting sea ice, and surface seawater. Our data highlight the complex TE biogeochemistry of late summer Arctic melt ponds and the variable importance of different sources for specific TEs. Dissolved TE concentrations indicate a strong influence from seawater intrusion for V, Ni, Cu, Cd, and Ba. Ultrafiltration methods reveal dissolved Fe, Zn, and Pb to be mostly colloidal (0.003–0.2 μm), while Mn, Co, Ni, Cu, and Cd are dominated by a truly soluble (<0.003 μm) fraction. Isotopically light dissolved Fe in some melt ponds suggests that photochemical and/or biologically driven redox cycling also takes place. Comparisons of particulate TE/Al ratios to mean crustal values indicate influences from lithogenic sources, including natural aerosols and/or sedimentary material, with significant enrichments for some elements, including Ni, Cu, Zn, Cd and Pb, that may result from anthropogenic aerosols, biogenic material, and/or in situ scavenging of dissolved TEs. Our results indicate that melt ponds represent a transitional environment in which some atmospherically-derived TEs undergo physical and/or chemical changes before their release to the surface ocean. As a result, the ongoing changes in sea ice areal extent, thickness, and melt season length are likely to influence the bioavailability of atmospheric TE input to the surface Arctic Ocean, with material released from snow and sea ice via melt ponds earlier in the summer and with more extensive direct deposition to the ocean surface.
•Melt pond contributions from snowmelt, sea ice melt and seawater are quantified.•Influence of the three source waters on melt pond dissolved TEs varies by element.•Melt ponds processes influence biogeochemistry of TEs released to surface ocean.
In September 2004 a detailed physical and chemical survey was conducted on an anticyclonic, cold‐core eddy located seaward of the Chukchi Shelf in the western Arctic Ocean. The eddy had a diameter of ...∼16 km and was centered at a depth of ∼160 m between the 1000 and 1500 m isobaths over the continental slope. The water in the core of the eddy (total volume of 25 km3) was of Pacific origin, and contained elevated concentrations of nutrients, organic carbon, and suspended particles. The feature, which likely formed from the boundary current along the edge of the Chukchi Shelf, provides a mechanism for transport of carbon, oxygen, and nutrients directly into the upper halocline of the Canada Basin. Nutrient concentrations in the eddy core were elevated compared to waters of similar density in the deep Canada Basin: silicate (+20 μmol L−1), nitrate (+5 μmol L−1), and phosphate (+0.4 μmol L−1). Organic carbon in the eddy core was also elevated: POC (+3.8 μmol L−1) and DOC (+11 μmol L−1). From these observations, the eddy contained 1.25 × 109 moles Si, 4.5 × 108 moles NO3−, 5.5 × 107 moles PO3−, 1.2 × 108 moles POC, and 1.9 × 109 moles DOC, all available for transport to the interior of the Canada Basin. This suggests that such eddies likely play a significant role in maintaining the nutrient maxima observed in the upper halocline. Assuming that shelf‐to‐basin eddy transport is the dominant renewal mechanism for waters of the upper halocline, remineralization of the excess organic carbon transported into the interior would consume 6.70 × 1010 moles of O2, or one half the total oxygen consumption anticipated arising from all export processes impacting the upper halocline.
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