Carbonate System Parameters Research Articles

Emergence of Summertime Hypoxia and Concurrent Carbonate Mineral Suppression in the Central Bohai Sea, China

AbstractThe Bohai Sea to the west of the North Yellow Sea in the Northwest Pacific is a semienclosed shallow‐water marginal sea on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism‐induced summertime dissolved oxygen (DO) decline and acidification, eight field surveys were conducted between August 2011 and September 2017, investigating seasonal and interannual variations in DO and carbonate system parameters in the central Bohai Sea. The historically lowest Bohai Sea DO was recorded at 66 μmol O2/kg in subsurface waters in early September 2015. By comparing our data set with literature data obtained in the 1980s, 1990s, and 2000s, the emergence history of summertime hypoxia in this marginal sea was shown. Associated with the seasonal DO shortage, the metabolism‐enhanced CO2 accumulation in subsurface waters in summer resulted in critical aragonite saturation states of close to or even less than 1.5, threatening calcifiers with potentially severe consequences for valuable shellfish fisheries. Based on model results combining the aerobic respiration (as indicated by apparent oxygen utilization) and the given wintertime/springtime atmospheric CO2 level of the time, this study suggests that combined stresses of the seasonal DO shortage and CO2 acidification capable of threatening Bohai Sea ecosystems could develop within decades.

Effects of seasonal upwelling and runoff on water chemistry and growth and survival of native and commercial oysters

AbstractThe effects of climate change, including ocean acidification and ocean heatwaves, on biological communities in estuaries are often uncertain. Part of the uncertainty is due to the complex suite of environmental factors in addition to acidification and warming that influence the growth of shells and skeletons of many estuarine organisms. The goal of this study was to document spatial and temporal variation in water column properties and to measure the in situ effects on larval and recently settled stages of ecologically important Olympia oysters (Ostrea lurida) and commercially important Pacific oysters (Crassostrea gigas) in a low‐inflow estuary with a Mediterranean climate in Northern California. Our results reveal that seasonal inputs of upwelled or riverine water create important and predictable gradients of carbonate system parameters, temperature, salinity, dissolved oxygen (DO), and other variables that influence oyster performance, and that the influence of these gradients is contingent upon the location in the estuary as well as seasonal timing. During upwelling events (dry season), temperature, carbonate chemistry, and DO had the greatest impact on oyster performance. During runoff events (wet season), gradients in salinity, nutrient concentrations, and total alkalinity driven by river discharge were comparatively more important. These results suggest that the spatial importance of carbonate chemistry and temperature are seasonally variable and are two of several other factors that determine oyster performance. We use these results to discuss future impacts on oysters given projected regional changes in the frequency and magnitude of upwelling and precipitation‐driven runoff events.

Dynamics of dissolved inorganic carbon and co2 fluxes between water and atmosphere in the main channel of the ob river

The results of investigation of the carbonate system parameters dynamics in the middle and lower reaches of the Ob River in July 2016 were analyzed. The distributions of the carbonate characteristics of the river waters were found to follow a steady latitudinal trend determined by the landscape-geochemical conditions of the drainage basin and by the permafrost distribution. The first field data on the magnitude and direction of the CO2 fluxes in the river water-atmosphere system in this region were presented. It was shown that the waters in the middle and lower reaches of the river are multiply supersaturated with CO2 with respect to its atmospheric content, and the daily average evasion reaches 625,9 mmol m-2 day-1 at an average value of 102,1 mmol m-2 day-1. It was determined that the waters of the Ob River in the studied reaches are a significant CO2 source to the atmosphere; according to conservative estimates, the total flux in July was 2 ⋅ 1011 g of carbon in the form of CO2.

Application of natural and artificial tracers to constrain CO2 leakage and degassing in the K-COSEM site, South Korea

Although Carbon Capture and Storage (CCS) has been demonstrated successfully on many occasions, the potential leakage of deep sequestrated CO2 into shallow groundwater remains a concern. To address this, an artificial injection experiment was performed at the K-COSEM test site in Eumseong, South Korea, that involved the release of CO2–infused water (16.9 kg of CO2 in 5 m3) containing He and Kr tracers into a shallow, heterogeneous, weathered-granite aquifer. The initial CO2–fluid was slightly oversaturated at the subsurface injection point, and thus the plume was expected to initially degas CO2 before equilibrating at in-situ conditions. Monitoring of carbonate system parameters in nearby observation wells helped define the evolution of the injected fluids, while the noble gas tracers were used to clearly define the physical behavior of the CO2 plume (including an estimate of degassed CO2 equal to 0.9–3.1%). This study demonstrates the potential use of noble gases for monitoring CO2 leakage in shallow aquifers, constraining mass balance and phase changes of leaking fluids, and better understanding local flow pathways. Furthermore, breakthrough of noble gases in this study was different from some previous experiments, suggesting that monitoring efficiency of these tracers may depend on leakage and site conditions.

Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system

Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

The carbonate system of the Eastern-most Mediterranean Sea, Levantine Sub-basin: Variations and drivers

The carbonate system is a vital buffering system that controls the pH of seawater and maintains a healthy environment for marine organisms. As concerns regarding the fate of anthropogenic CO2 in the oceans are rising, it is becoming increasingly urgent to systematically quantify and understand this system's parameters, particularly in heavily human-impacted areas, such as the Mediterranean Sea. To date, the paucity of time-series stations adopted to monitor the carbonate system in this sea has precluded characterizing the region at an adequate spatial resolution. Here, we present and study the seasonal and annual variability and drivers of the first carbonate system dataset collected for the Lebanese waters, monthly at the upper 80 m between 2012 and 2017 in two time-series stations offshore the North of Lebanon-Levantine Sub-basin, Eastern Mediterranean Sea. Annual trends were calculated for the non-adjusted and the adjusted carbonate system parameters (an adjustment that reduces the influence of simple-dilution-concentration [SDC] processes on the trends). Our results show high carbonate system inventory [total alkalinity (AT), total dissolved inorganic carbon (CT), and pH] compared to other Mediterranean areas. The obtained trends reflect increasing rates for both AT and CT, only significant at surface for CT (+5 ± 2 μmol kg−1.yr−1; p < 0.05). Whereas annual acidification rates were always significant (i.e. from −0.009 ± 0.004 to −0.0021 ± 0.001 pH units.yr−1 at 0 m and in the upper 80 m respectively for pHT25adj). Concomitantly, decreasing trends of the saturation states for both CaCO3 minerals were calculated (−0.1 ± 0.04 and −0.07 ± 0.02 yr−1 for calcite and aragonite respectively at surface; p < 0.05). Moreover, our results showed that SDC processes, together with CO2 release/invasion and the active overturning circulation, are controlling this system in the Lebanese seawater, Eastern-most Mediterranean Sea. Contrariwise, the increasing trend of total alkalinity, mainly attributed to SDC processes (i.e. riverine inputs, weathering during extreme events, precipitations), may be buffering the observed acidification rate, which could have been worst in case AT in our area was lower. Highlights•The first carbonate system dataset collected for the Lebanese waters, -Levantine Sub-basin, Eastern Mediterranean Sea, are presented in this study.•Our results show high carbonate system inventory compared to other Mediterranean areas with increasing rates of both AT and CT.•Annual acidification rates have been document (i.e. from −0.009 ± 0.004 to −0.0021 ± 0.001 pH units.yr−1 at 0 m and in the upper 80 m respectively for pHT25adj.•Decreasing trends of the saturation states for both CaCO3 minerals were calculated.•Simple-dilution-concentration processes, together with CO2 release/invasion and the active overturning circulation, are controlling this system in our study area.•The increasing trend of total alkalinity may be buffering the observed acidification rate, which could have been worst in case AT in our area was lower.

Remotely Forced Decadal Physical and Biogeochemical Variability of North Pacific Subtropical Mode Water Over the Last 40 Years

AbstractHalf‐century‐long observations at the 137°E repeat hydrographic section across the western North Pacific have been analyzed to demonstrate remotely forced decadal physical and biogeochemical variability of Subtropical Mode Water (STMW) over the last 40 years. During unstable periods of the Kuroshio Extension (KE) that lagged the warm phase of the Pacific Decadal Oscillation by 3–4 years, high regional eddy activity reduced the formation rate and salinity of STMW in its main formation region south of the KE. At the 137°E section south of Japan, decreasing southwestward advection of oxygen‐rich STMW from the formation region resulted in decreases of its cross‐sectional area, dissolved oxygen, pH, and aragonite saturation state and increases of nutrients and dissolved inorganic carbon, among which changes of the carbon system parameters accelerated their long‐term trends. Such changes reversed and acidification slowed down during stable‐KE periods, especially in the current period since 2010 exhibiting a hiatus of acidification.

Carbon Dynamics During the Formation of Sea Ice at Different Growth Rates

Controlled laboratory experiments have shed new light on the potential importance of brine rejection during sea-ice formation for carbon dioxide sequestration in the ocean. We grew ice in an experimental seawater tank (1 m3) under abiotic conditions at three different air temperatures (-40 °C, -25 °C, -15 °C) to determine how different ice growth rates affect the allocation of carbon to ice, water, or air. Carbonate system parameters were determined by discrete sampling of ice cores and water, as well as continuous measurements by multiple sensors deployed mainly in the water phase. A budgetary approach revealed that of the initial total inorganic carbon (TIC) content of the water converted to ice, only 28-29 % was located in the ice phase by the end of the experiments run at the warmest temperature, whereas for the coldest ambient temperature, 46-47 % of the carbon remained in the ice. Exchange with air appeared to be negligible, with the majority of the TIC remaining in the under-ice water (53 -72 %). Along with a good correlation between salinity and TIC in the ice and water samples, these observations highlight the importance of brine drainage to TIC redistribution during ice formation. For experiments without mixing of the under-ice water, the sensor data further suggested stronger stratification, likely related to release of denser brine, and thus potentially larger carbon sequestration for ice grown at a colder temperature and faster growth rate.

The Marine Carbonate System at Maluku and Sulawesi Seas

Maluku and Sulawesi Sea s are parts of Indonesian Large Marine Ecosystem in the east area which has an important role for Indonesian fisheries activities. Those areas are facing the climate change phenomena that could change their marine ecosystem quality. Marine carbonate system is very important for controlling the circulation of CO 2 between the atmosphere and the ocean as well as regulating the pH value of waters. We conducted in situ measurement of marine carbonate system parameters (pH, Total Alkalinity, Total CO 2 (TCO 2 ) and pCO 2 ) during IJEP cruise on September 2016 to analyses spatial variations of the marine carbonate system in the Northwest Monsoon period. The results of marine carbonate system showed that in general Maluku and Sulawesi Sea s are under super saturation of CO 2 or acts as source of CO 2 which have range value of sea surface partial of CO 2 (pCO 2 ) between 280 – 500 µatm. Spatial distribution of sea surface pH values measured with optical sensor at study sites showed varied between 8,03 – 8,15. Measurement results of Total alkalinity (TA) by using titration method varied between 2300 – 2400 µmol/Kg meanwhile range value of TCO 2 was 2300 – 2500 µmol/Kg at surface waterand it’s increased through the deep water with the range value of TCO 2 was 2500 – 2800 µmol/Kg at deeper layer of water.Characteristic of marine carbonate system between Maluku and Sulawesi Sea s was different in the Southeast monsoon period indicates complex marine biogeochemistry processes influences those system .

Rapid warming and salinity changes in the Gulf of Maine alter surface ocean carbonate parameters and hide ocean acidification

A profound warming event in the Gulf of Maine during the last decade has caused sea surface temperatures to rise to levels exceeding any earlier observations recorded in the region over the last 150 years. This event dramatically affected CO2 solubility and, in turn, the status of the sea surface carbonate system. When combined with the concomitant increase in sea surface salinity and assumed rapid equilibration of carbon dioxide across the air sea interface, thermodynamic forcing partially mitigated the effects of ocean acidification for pH, while raising the saturation index of aragonite (varOmega_{AR}) by an average of 0.14 U. Although the recent event is categorically extreme, we find that carbonate system parameters also respond to interannual and decadal variability in temperature and salinity, and that such phenomena can mask the expression of ocean acidification caused by increasing atmospheric carbon dioxide. An analysis of a 34-year salinity and SST time series (1981–2014) shows instances of 5–10 years anomalies in temperature and salinity that perturb the carbonate system to an extent greater than that expected from ocean acidification. Because such conditions are not uncommon in our time series, it is critical to understand processes controlling the carbonate system and how ecosystems with calcifying organisms respond to its rapidly changing conditions. It is also imperative that regional and global models used to estimate carbonate system trends carefully resolve variations in the physical processes that control CO2 concentrations in the surface ocean on timescales from episodic events to decades and longer.

Robust Constraints on Past CO2 Climate Forcing From the Boron Isotope Proxy

AbstractThe atmospheric concentration of the greenhouse gas carbon dioxide, CO2, is intimately coupled to the carbon chemistry of seawater, such that the radiative climate forcing from CO2 can be changed by an array of physical, geochemical, and biological ocean processes. For instance, biological carbon sequestration, seawater cooling, and net CaCO3 dissolution are commonly invoked as the primary drivers of CO2 change that amplify the orbitally paced ice age cycles of the late Pleistocene. Based on first‐principle arguments with regard to ocean chemistry, we demonstrate that seawater pH change (ΔpH) is the dominant control that effectively sets CO2 radiative forcing (ΔF) on orbital timescales, as is evident from independent late Pleistocene reconstructions of pH and CO2. In short, all processes relevant for CO2on orbital timescales, including temperature change, cause pH to change to bring about fractional CO2 change so as to yield a linear relationship of ΔpH to CO2 climate forcing. Further, we show that ΔpH and CO2 climate forcing can be reconstructed using the boron isotope pH proxy more accurately than absolute pH or CO2, even if seawater boron isotope composition is poorly constrained and without information on a second carbonate system parameter. Thus, our formalism relaxes otherwise necessary assumptions to allow the accurate determination of orbital timescale CO2 radiative forcing from boron isotope pH reconstructions alone, thereby eliminating a major limitation of current methods to estimate our planet's climate sensitivity from the geologic record.

Seasonal carbonate chemistry variability in marine surface waters of the US Pacific Northwest

Abstract. Fingerprinting ocean acidification (OA) in US West Coast waters is extremely challenging due to the large magnitude of natural carbonate chemistry variations common to these regions. Additionally, quantifying a change requires information about the initial conditions, which is not readily available in most coastal systems. In an effort to address this issue, we have collated high-quality publicly available data to characterize the modern seasonal carbonate chemistry variability in marine surface waters of the US Pacific Northwest. Underway ship data from version 4 of the Surface Ocean CO2 Atlas, discrete observations from various sampling platforms, and sustained measurements from regional moorings were incorporated to provide ∼ 100 000 inorganic carbon observations from which modern seasonal cycles were estimated. Underway ship and discrete observations were merged and gridded to a 0.1° × 0.1° scale. Eight unique regions were identified and seasonal cycles from grid cells within each region were averaged. Data from nine surface moorings were also compiled and used to develop robust estimates of mean seasonal cycles for comparison with the eight regions. This manuscript describes our methodology and the resulting mean seasonal cycles for multiple OA metrics in an effort to provide a large-scale environmental context for ongoing research, adaptation, and management efforts throughout the US Pacific Northwest. Major findings include the identification of unique chemical characteristics across the study domain. There is a clear increase in the ratio of dissolved inorganic carbon (DIC) to total alkalinity (TA) and in the seasonal cycle amplitude of carbonate system parameters when moving from the open ocean North Pacific into the Salish Sea. Due to the logarithmic nature of the pH scale (pH = −log10[H+], where [H+] is the hydrogen ion concentration), lower annual mean pH values (associated with elevated DIC : TA ratios) coupled with larger magnitude seasonal pH cycles results in seasonal [H+] ranges that are ∼ 27 times larger in Hood Canal than in the neighboring North Pacific open ocean. Organisms living in the Salish Sea are thus exposed to much larger seasonal acidity changes than those living in nearby open ocean waters. Additionally, our findings suggest that lower buffering capacities in the Salish Sea make these waters less efficient at absorbing anthropogenic carbon than open ocean waters at the same latitude.All data used in this analysis are publically available at the following websites: Surface Ocean CO2 Atlas version 4 coastal data, https://doi.pangaea.de/10.1594/PANGAEA.866856 (Bakker et al., 2016a);National Oceanic and Atmospheric Administration (NOAA) West Coast Ocean Acidification cruise data, https://doi.org/10.3334/CDIAC/otg.CLIVAR_NACP_West_Coast_Cruise_2007 (Feely and Sabine, 2013); https://doi.org/10.7289/V5JQ0XZ1 (Feely et al., 2015b); https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0157445 (Feely et al., 2016a); https://doi.org/10.7289/V5C53HXP (Feely et al., 2015a);University of Washington (UW) and Washington Ocean Acidification Center cruise data, https://doi.org/10.5281/zenodo.1184657 (Fassbender et al., 2018);Washington State Department of Ecology seaplane data, https://doi.org/10.5281/zenodo.1184657 (Fassbender et al., 2018);NOAA Moored Autonomous pCO2 (MAPCO2) buoy data, https://doi.org/10.3334/CDIAC/OTG.TSM_LAPUSH_125W_48N (Sutton et al., 2012); https://doi.org/10.3334/CDIAC/OTG.TSM_WA_125W_47N (Sutton et al., 2013); https://doi.org/10.3334/CDIAC/OTG.TSM_DABOB_122W_478N (Sutton et al., 2014a); https://doi.org/10.3334/CDIAC/OTG.TSM_TWANOH_123W_47N (Sutton et al., 2016a);UW Oceanic Remote Chemical/Optical Analyzer buoy data, https://doi.org/10.5281/zenodo.1184657 (Fassbender et al., 2018);NOAA Pacific Coast Ocean Observing System cruise data, https://doi.org/10.5281/zenodo.1184657 (Fassbender et al., 2018).

Seawater acidification and anthropogenic carbon distribution on the continental shelf and slope of the western South Atlantic Ocean

Abstract Increases in atmospheric carbon dioxide (CO2) concentrations since the beginning of the Industrial Revolution have led to increases in the flux of CO2 into the oceans. A large fraction of this increased flux occurs due to anthropogenic carbon (Cant), i.e., the excess carbon released by human activities. The oceans take up large amounts of Cant, and this process likely affects calcifying organisms and the formation and dissolution of calcium carbonate. In this study, we measured hydrographic and carbonate system parameters in the western South Atlantic Ocean. This region is key for understanding climate issues; however, it remains understudied, especially regarding the drivers of ocean acidification and its effects on marine organisms and ecosystems. Using novel data collected in this undersampled area of the global ocean, we (i) determined values of the marine carbonate system parameters and (ii) characterized the seawater acidification state along the western boundary current off the southeastern and southern coasts of Brazil. Specifically, we measured total alkalinity (AT) and pH and applied the TrOCA approach (to derive the Cant storage in the water masses) in the study area. Our results show that (i) AT and the dissolved inorganic carbon distribution in the study region are governed by calcification/dissolution and dilution/evaporation processes and that (ii) Cant is present in the central and intermediate water masses at an average concentration (± method precision) of 75.0 ± 6.6 μmol kg−1 and at depths of up to ~1000 m. The highest concentrations are observed in the lower levels of the Tropical Water, although we cannot exclude an overestimated Cant concentration in this layer, and the South Atlantic Central Water (SACW). These findings suggest that cross-shelf processes likely contribute to increased Cant along western boundary current systems. The storage of Cant in the SACW and the Antarctic Intermediate Water has led to long-term pH decreases of −0.17 ± 0.07 and −0.10 ± 0.06 pH units, respectively.

Does photosynthesis provoke formation of shell deformity in wild mytilid mussels infested with green microalgae Coccomyxa? – A conceptual model and research agenda

This report proposes a conceptual model for the formation of L-shaped shell deformity in wild mytilid mussels Mytilus spp. highly infested by unicellular photosynthetic microalgae Coccomyxa sp. where a key role plays the influence of the alga's photosynthesis on the parameters of carbonate system at the site of calcification. A number of research questions are posed, to be investigated through a combination of experimental methods.

The role of local environmental changes on maerl and its associated non-calcareous epiphytic flora in the Bay of Brest

Large stands of free living (calcareous) coralline algae — called maerl beds — play a major role as ecosystem engineers in coastal areas throughout the world. They are also subject to strong anthropogenic pressures at global and local scales, which threaten their survival. However, the macroalgal epiphytes growing on maerl may benefit from these pressures, developing to the detriment of maerl algae. Here, we sought to gain insight into how maerl beds and their epiphytic algae are disturbed by variations in the local environment, and how these variations affect their capacity to respond to global change. In 2015, we monitored three maerl beds located in the Bay of Brest (Brittany, France). Sites with contrasting conditions were selected, with one station lying in a zone close to the harbor (northern basin S1) and two stations (S2 and S3) located in areas away from the main urban effluents but subject to other sources of local change: higher currents at S3 (PREVIMER Ocean Forecast) and higher sedimentation rates at S2 (Ehrhold et al., 2016). We observed significant temporal variations of physico-chemical parameters, on an annual but also on a daily basis. Results showed that S2 differentiated itself from the other stations, this station experienced higher fluctuations of salinity, nutrient concentrations and carbonate system parameters and hosted the lowest (living) maerl biomass (4.38 ± 1.54 kg DW m−2). S3 observed the highest living maerl biomass (14.56 ± 1.61 kg DW m−2) and the lowest non-calcareous epiphytic macroalgal abundance (0.1–7.9 g DW m−2). S1 displayed the highest heterogeneity in terms of living maerl biomass (it varied from 0.8 to 8.6 kg DW m−2), and the highest Chl a content. However, we did not record differences in terms of physico-chemical parameters between S1 and S3. No positive relationship was observed between nutrient enrichment and macroalgal epiphyte abundance, but epiphyte abundance was higher at stations with lower maerl biomass (S1 and S2) (mean value ranged from 4.6 to 49.0 g DW m−2 at S1 and from 7.4 to 23.7 g DW m−2 at S2). Our results highlight the importance of local changes on the development, survival and capacity to adapt to global change of maerl beds.

Carbonate system parameters of an algal-dominated reef along West Maui

Abstract. Constraining coral reef metabolism and carbon chemistry dynamics are fundamental for understanding and predicting reef vulnerability to rising coastal CO2 concentrations and decreasing seawater pH. However, few studies exist along reefs occupying densely inhabited shorelines with known input from land-based sources of pollution. The shallow coral reefs off Kahekili, West Maui, are exposed to nutrient-enriched, low-pH submarine groundwater discharge (SGD) and are particularly vulnerable to the compounding stressors from land-based sources of pollution and lower seawater pH. To constrain the carbonate chemistry system, nutrients and carbonate chemistry were measured along the Kahekili reef flat every 4 h over a 6-day sampling period in March 2016. Abiotic process – primarily SGD fluxes – controlled the carbonate chemistry adjacent to the primary SGD vent site, with nutrient-laden freshwater decreasing pH levels and favoring undersaturated aragonite saturation (Ωarag) conditions. In contrast, diurnal variability in the carbonate chemistry at other sites along the reef flat was driven by reef community metabolism. Superimposed on the diurnal signal was a transition during the second sampling period to a surplus of total alkalinity (TA) and dissolved inorganic carbon (DIC) compared to ocean endmember TA and DIC measurements. A shift from positive net community production and positive net community calcification to negative net community production and negative net community calcification was identified. This transition occurred during a period of increased SGD-driven nutrient loading, lower wave height, and reduced current speeds. This detailed study of carbon chemistry dynamics highlights the need to incorporate local effects of nearshore oceanographic processes into predictions of coral reef vulnerability and resilience.

Over-calcified forms of the coccolithophore &amp;lt;i&amp;gt;Emiliania huxleyi&amp;lt;/i&amp;gt; in high-CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; waters are not preadapted to ocean acidification

Abstract. Marine multicellular organisms inhabiting waters with natural high fluctuations in pH appear more tolerant to acidification than conspecifics occurring in nearby stable waters, suggesting that environments of fluctuating pH hold genetic reservoirs for adaptation of key groups to ocean acidification (OA). The abundant and cosmopolitan calcifying phytoplankton Emiliania huxleyi exhibits a range of morphotypes with varying degrees of coccolith mineralization. We show that E. huxleyi populations in the naturally acidified upwelling waters of the eastern South Pacific, where pH drops below 7.8 as is predicted for the global surface ocean by the year 2100, are dominated by exceptionally over-calcified morphotypes whose distal coccolith shield can be almost solid calcite. Shifts in morphotype composition of E. huxleyi populations correlate with changes in carbonate system parameters. We tested if these correlations indicate that the hyper-calcified morphotype is adapted to OA. In experimental exposures to present-day vs. future pCO2 (400 vs. 1200 µatm), the over-calcified morphotypes showed the same growth inhibition (−29.1±6.3 %) as moderately calcified morphotypes isolated from non-acidified water (−30.7±8.8 %). Under the high-CO2–low-pH condition, production rates of particulate organic carbon (POC) increased, while production rates of particulate inorganic carbon (PIC) were maintained or decreased slightly (but not significantly), leading to lowered PIC ∕ POC ratios in all strains. There were no consistent correlations of response intensity with strain origin. The high-CO2–low-pH condition affected coccolith morphology equally or more strongly in over-calcified strains compared to moderately calcified strains. High-CO2–low-pH conditions appear not to directly select for exceptionally over-calcified morphotypes over other morphotypes, but perhaps indirectly by ecologically correlated factors. More generally, these results suggest that oceanic planktonic microorganisms, despite their rapid turnover and large population sizes, do not necessarily exhibit adaptations to naturally high-CO2 upwellings, and this ubiquitous coccolithophore may be near the limit of its capacity to adapt to ongoing ocean acidification.

Spectrophotometric calibration procedures to enable calibration-free measurements of seawater calcium carbonate saturation states

A simple protocol was developed to measure seawater calcium carbonate saturation states (Ωspec) spectrophotometrically. Saturation states are typically derived from the separate measurement of two other carbon system parameters, with each requiring unique instrumentation and often complex measurement protocols. Using the new protocol, the only required equipment is a thermostatted laboratory spectrophotometer. For each seawater sample, spectrophotometric measurements of pH (visible absorbance) are made in paired optical cells, one with and one without added nitric acid. Ultraviolet absorbance is measured to determine the amount of added acid based on the direct proportionality between nitrate concentration and UV absorbance. Coupled measurements of pH and the alkalinity change that accompanies the nitric acid addition allow calculation of a seawater sample's original carbonate ion concentration and saturation state. These paired absorbance measurements yield Ωspec (and other carbonate system parameters), with each sample requiring about 12 min processing time. Initially, an instrument-specific nitrate molar absorptivity coefficient must be determined (due to small but significant discrepancies in instrumental wavelength calibrations), but thereafter no further calibration is needed. In this work, the 1σ precision of replicate measurements of aragonite saturation state was found to be 0.020, and the average difference between Ωspec and Ω calculated conventionally from measured total alkalinity and pH (Ωcalc) was −0.11% ± 0.96% (a level of accuracy comparable to that obtained from spectrophotometric measurements of carbonate ion concentration). Over the entire range of experimental conditions, 0.97 < Ω < 3.17 (n = 125), all measurements attained the Global Ocean Acidification Observing Network's “weather level” goal for accuracy and 90% attained the more stringent “climate level” goal. When Ωspec was calculated from averages of duplicate samples (n = 56), the precision improved to 0.014 and the average difference between Ωspec and Ωcalc improved to −0.11% ± 0.73%. Additionally, 97% of the duplicate-based Ωspec measurements attained the “climate level” accuracy goal. These results indicate that the simple measurement protocol developed in this work should be widely applicable for monitoring fundamental seawater changes associated with ocean acidification.

Arctic Ocean benthic foraminifera Mg/Ca ratios and global Mg/Ca-temperature calibrations: New constraints at low temperatures

We explore the use of Mg/Ca ratios in six Arctic Ocean benthic foraminifera species as bottom water palaeothermometers and expand published Mg/Ca-temperature calibrations to the coldest bottom temperatures (<1 °C). Foraminifera were analyzed in surface sediments at 27 sites in the Chukchi Sea, East Siberian Sea, Laptev Sea, Lomonosov Ridge and Petermann Fjord. The sites span water depths of 52–1157 m and bottom water temperatures (BWT) of −1.8 to +0.9 °C. Benthic foraminifera were alive at time of collection, determined from Rose Bengal (RB) staining. Three infaunal and three epifaunal species were abundant enough for Mg/Ca analysis. As predicted by theory and empirical evidence, cold water Arctic Ocean benthic species produce low Mg/Ca ratios, the exception being the porcelaneous species Quinqueloculina arctica. Our new data provide important constraints at the cold end (<1 °C) when added to existing global datasets. The refined calibrations based on the new and published global data appear best supported for the infaunal species Nonionella labradorica (Mg/Ca = 1.325 ± 0.01 × e^(0.065 ± 0.01 × BWT), r2 = 0.9), Cassidulina neoteretis (Mg/Ca = 1.009 ± 0.02 × e^(0.042 ± 0.01 × BWT), r2 = 0.6) and Elphidium clavatum (Mg/Ca = 0.816 ± 0.06 + 0.125 ± 0.05 × BWT, r2 = 0.4). The latter is based on the new Arctic data only. This suggests that Arctic Ocean infaunal taxa are suitable for capturing at least relative and probably semi-quantitative past changes in BWT. Arctic Oridorsalis tener Mg/Ca data are combined with existing O. umbonatus Mg/Ca data from well saturated core-tops from other regions to produce a temperature calibration with minimal influence of bottom water carbonate saturation state (Mg/Ca = 1.317 ± 0.03 × e^(0.102 ± 0.01 BWT), r2 = 0.7). The same approach for Cibicidoides wuellerstorfi yields Mg/Ca = 1.043 ± 0.03 × e^(0.118 ± 0.1 BWT), r2 = 0.4. Mg/Ca ratios of the porcelaneous epifaunal species Q. arctica show a clear positive relationship between Mg/Ca and Δ[CO32−] indicating that this species is not suitable for Mg/Ca-palaeothermometry at low temperatures, but may be useful in reconstructing carbonate system parameters through time.

Boron isotope-based seasonal paleo-pH reconstruction for the Southeast Atlantic – A multispecies approach using habitat preference of planktonic foraminifera

The boron isotopic composition of planktonic foraminiferal shell calcite (δ11BCc) provides valuable information on the pH of ambient water at the time of calcification. Hence, δ11BCc of fossil surface-dwelling planktonic foraminifera can be used to reconstruct ancient aqueous pCO2 if information on a second carbonate system parameter, temperature and salinity is available. However, pH and pCO2 of surface waters may vary seasonally, largely due to changes in temperature, DIC, and alkalinity. As also the shell fluxes of planktonic foraminifera show species-specific seasonal patterns that are linked to intra-annual changes in temperature, it is obvious that δ11BCc of a certain species reflects the pH and thus pCO2 biased towards a specific time period within a year. This is important to consider for the interpretation of fossil δ11BCc records that may mirror seasonal pH signals. Here we present new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) δ11BCc coretop data for the planktonic foraminifera species Globigerina bulloides, Globigerinoides ruber, Trilobatus sacculifer and Orbulina universa and compare them with δ11Bborate derived from seasonally resolved carbonate system parameters. We show that the inferred season-adjusted δ11BCc/δ11Bborate relationships are similar to existing calibrations and can be combined with published δ11BCc field and culture data to augment paleo-pH calibrations. To test the applicability of these calibrations, we used a core drilled on the Walvis Ridge in the Southeast Atlantic spanning the last 330,000 years to reconstruct changes in surface-water pCO2. The reconstruction based on G. bulloides, which reflects the austral spring season, was shown to yield values that closely resemble the Vostok ice-core data indicating that surface-water pCO2 was close to equilibrium with the atmosphere during the cooler spring season. In contrast, pCO2 estimated from δ11BCc of O. universa, T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up to ∼50 ppmv higher values than the Vostok ice-core data. This is probably due to the higher austral summer and fall temperatures, as shown by Mg/Ca to be on average ∼4 °C higher than during the cooler spring season, accounting for an increase in pCO2 of ∼4% per 1 °C. Our results demonstrate that paleo-pH estimates based on δ11BCc contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species.