δ13C Of Dissolved Inorganic Carbon Research Articles

Mixing Dynamics Within the Kuroshio Area Are Reflected in Dissolved Inorganic Radiocarbon Values

AbstractKuroshio is an important western boundary current system in the North Pacific subtropical gyre. Mesoscale eddies play an important role in Kuroshio path variations, which significantly affect fisheries, marine navigation, and climate in regions along the Kuroshio path. However, the direct physical impacts of the Kuroshio variabilities on mixing dynamics within the water columns off the southern coast of Japan remain unclear due to the lack of observational records. The radiocarbon (Δ14C) in dissolved inorganic carbon (DIC) of seawater has been used as a reliable tracer of water circulation and mixing processes. Here, we present the high‐resolution dissolved inorganic radiocarbon (DIC ∆14C) measurements for water samples collected during the Kuroshio large‐meander (LM) period in March 2022 to observe the mixing dynamics within the Kuroshio area and elucidate the controls of mixing processes. In the present study, horizontal variations (12–175‰) in DIC Δ14C values were observed between 400 and 1,000 m and were attributed mainly to changes in the depth of isopycnal surfaces associated with mesoscale eddies and the position of the Kuroshio axis. Furthermore, fluctuations in DIC Δ14C values (10–25‰) were observed on the same isopycnal surfaces ( 25.5–27.0), suggesting the occurrence of diapycnal mixing. By comparing this newly obtained data with values previously reported at similar locations, variations in penetration depths of high bomb 14C signals from past observations are found to be mainly caused by changes in the depth of isopycnal surfaces associated with mesoscale eddies and Kuroshio path variations.

Simple bulk pigment analysis suggests microphytobenthos contributions to food webs may be underestimated due to isotopic contamination by remineralized wetland carbon

Stable isotope studies have revealed the importance of microphytobenthos (MPB) in coastal food webs. Microalgae typically have δ13C values between depleted C3 wetland/terrestrial macrophytes, and enriched C4 macrophytes and seagrasses. However, the challenges of obtaining clean samples of microalgae from sediments means they are often represented by limited sampling in many food web studies; consequently, we have a limited understanding of spatial and temporal variation in their δ13C values. We tested a simple technique to measure the δ13C of bulk pigments extracted from surficial sediments to represent MPB and applied it to quantify fine scale spatial variation in MPB δ13C around salt marshes. The bulk extraction method is logistically simple, and drives substantial but relatively consistent fractionation in δ13C of −3.5 ± 0.13 ‰ (mean ± 1 S.E., range = 2.3–4.4 ‰, n = 18 paired comparisons) compared to whole cell values. The consistency in fractionation suggests that spatial and temporal δ13C patterns measured in field samples should reflect real variation in source values, and that measured values could be corrected and incorporated into isotope mixing models. In 88 MPB samples among four marsh sites over two summers, MPB δ13C in marsh creeks was lower by an average of 4.4 ± 0.72 ‰ and up to 8.4 ‰ compared to sites along the outer marsh-open water fringe 10's of m away. Few food web studies incorporate this magnitude of variation in their MPB source estimates into mixing models. Over three weekly samplings at one marsh creek site, low tide dissolved inorganic carbon (DIC) δ13C was similarly lower by 4.8 ± 0.36 ‰ and up to 6.2 ‰ compared to high tide and adjacent open water DIC values. The significant small-scale variability in MPB δ13C appears to be driven by remineralized marsh carbon which depletes the DIC δ13C in the marsh creeks, a phenomenon that has long been recognized but is rarely considered in food web studies. Mixing models that assume a narrower range in MPB source values will erroneously attribute isotopically variable MPB contributions to end-member production sources thereby clouding our understanding of energy flows through coastal seascapes.

Changes in Dissolved Inorganic Carbon Across Yangtze River Regulated by Dam and River‐Lake Exchange

AbstractThe boom in dam construction and continuous river‐lake exchange has had a profound impact on the transmission and transformation of riverine dissolved inorganic carbon (DIC). An in‐depth understanding of the change mechanisms of DIC concentrations and sources driven by dam operation and lake recharge is crucial for regulating greenhouse gas emissions and evaluating the impact of DIC on the global carbon cycle. This study investigated dam‐ and lakes‐driven DIC via the concentration and δ13C of DIC, combined with anions, cations, δD and δ18O in the main stream of the Yangtze River. DIC showed a decreasing trend from upper reach (2,262.31 ± 113.69 μmol kg−1) to lower reach (1,771.61 ± 89.36 μmol kg−1). Carbonate dissolution proportion (from 36.45% to 28.44%) and atmospheric CO2 proportion (from 37.51% to 22.94%) of DIC decreased from upper reach to lower reach, whereas soil CO2 proportion of DIC (from 26.01% to 48.63%) increased. The control of dam operation on DIC biogeochemical process was revealed from different time scales. From the perspective of short‐term seasonal changes (from 2020 to 2021), the mineralization of organic matter in the dry season strengthened CO2 degassing and calcite precipitation, reducing the DIC and increasing the proportion of soil CO2. Meanwhile, longer periods of runoff retention provided sufficient time for water–rock reactions in the wet season and increased the DIC and carbonate dissolution source in the reservoir area. On a long‐term scale (from 2009 to 2021), a decrease in pH driven by sediment mineralization contributed to an annual increase in DIC in the reservoir. The flow of lakes mixed into the mainstream was revealed by the enrichment of δ18O, and river‐communicating recharge decreased the DIC and carbonate dissolution source. We show that dam operation and lake inflow change DIC concentrations and sources and therefore need to be considered in the transmission and transformation processes of DIC in the river‐ocean continuum.

Origin of Deep Methane from Active Faults along the Itoigawa–Shizuoka Tectonic Line between the Eurasian and North American Plates: 13C/12C and 14C/12C Methane Profiles from a Pull-Apart Basin at Lake Suwa

Stable and radiocarbon isotope analyses provide valuable insights into carbon dynamics in hydrosphere ecosystems. A lake containing seeps of deep methane (CH4) may provide an ideal environment for elucidating such dynamics, including the involvement of deep carbon in the surface ecosystem and its food web. In this study, we performed carbon isotope analysis of seep gas, lake water, and a cyanobacterial bloom sample collected at Lake Suwa, a pull-apart basin on the Itoigawa–Shizuoka tectonic line, Japan. CH4 collected from two seep sites 100 m from the lakeshore was 14C-depleted (Δ14C = −972.1 to −989.8‰), indicating that the origins of seep CH4 and benthic bubble CH4 (Δ14C = +21.7‰) collected near the lakeshore differed. The results suggested that benthic bubble CH4 was produced by methanogenic archaea in benthic sediments and that seep CH4 was produced by methanogenic archaea using relic carbon. The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14C = −103.1 to −630.6‰) and near the lakeshore (Δ14C = −100.9‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. The Δ14C values of the cyanobacterial bloom sample also showed values similar to those of DIC (Δ14C = −110.3‰), suggesting that 10–63% of DIC in surface water and 10% in primary producers originated from relic carbon, including CH4 and CO2, and was incorporated into the surface ecosystem of Lake Suwa.

Using Radiocarbon Measurements of Dissolved Inorganic Carbon to Determine a Revised Residence Time for Deep Baffin Bay

The Canadian Arctic is warming at three times the rate of the rest of the planet and the effects of climate change on the Arctic marine carbon cycle remains unconstrained. Baffin Bay is a semi-enclosed, Arctic basin that connects the Arctic Ocean to the north to the Labrador Sea to the south. While the physical oceanography of surface Baffin Bay is well characterized, less is known about deep water formation mechanisms within the Basin. Only a few residence-time estimates for Baffin Bay Deep Water (BBDW) exist and range from 20 to 1450 years. Better residence time estimates are needed to understand the oceanographic significance of Baffin Bay. Here we report stable carbon (δ13C) and radiocarbon (Δ14C) values of dissolved inorganic carbon (DIC) collected aboard the CCGS Amundsen in 2019. DIC δ13C and Δ14C values between ranged between -0.7‰ to +1.9‰ and -90.0‰ to +29.8‰, respectively. Surface DIC δ13C values were between +0.7‰ to +1.9‰, while deep (>100m) values were 0.0 to -0.7‰. Surface DIC Δ14C values ranged between -5.4‰ to +22.9‰, while deep DIC (>1400m) DIC Δ14C averaged -82.2 ± 8.5‰ (n = 9). To constrain natural DIC Δ14C values, we quantified the amount of atmospheric “bomb” 14C in DIC (Δ14Cbomb; using the potential alkalinity method; Palk) and anthropogenic DIC (DICanth; using the ΔC* method). Both proxies indicate an absence of Δ14Cbomb and DICanth below 1000m. Using two previously proposed deep water formation mechanisms and our corrected DIC Δ14Cnatural values, we estimated a 14C-based residence time of 360-690 years for BBDW. Based on these residence times, we infer carbon is likely stored for centuries in deep Baffin Bay.

Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air‐Sea CO2 Exchange Rate Tracing

AbstractΔ14C values of the atmosphere and seawater dissolved inorganic carbon (DIC) were measured during a cruise in the South China Sea (SCS) in September 2015, in order to determine the 14C flux and bomb 14C‐based air‐sea CO2 exchange rates for this region. The background atmospheric Δ14C value (13.8 ± 5.0‰) for the SCS during that period was lower than that (35.4 ± 3.4‰) of surface seawater (5 m) DIC, and a net transfer of 14C from the sea to the atmosphere (7.4 ± 5.0 × 1011 atoms m−2 yr−1) was determined at the wind speed of 5.2 ± 1.7 m s−1. Seawater DIC Δ14C profiles showed the highest value (37.9 ± 3.7‰) at a depth of 100 m, a rapid decrease below that depth to −220.3 ± 3.2‰ at 1,500 m, and nearly constant values below 1,500 m. The average mean penetration depth of bomb 14C was 585.5 ± 99.2 m, and a value of 8.2 ± 1.0 × 109 atoms cm−2 was obtained for the bomb 14C inventory in this region. Based on this inventory, a long‐term (1954–2015) average air‐sea CO2 exchange rate of 20.2 ± 2.8 mol m−2 yr−1 was traced for the SCS. Combined with the pCO2 measurements in this region, a net CO2 flux rate of 0.54 ± 0.08 mol m−2 yr−1 was yielded for the SCS, which is comparable to the cruise measured flux (0.44 ± 0.62 mol m−2 yr−1) obtained from a synthesis study (Li et al., 2020, https://doi.org/10.1016/j.pocean.2020.102272). Our study highlights the importance of continued atmospheric and seawater 14C observations on determining the air‐sea flux in this region.

Carbon dynamics and CO2 and CH4 exchange in the mangrove dominated Guayas river delta, Ecuador

Although estuaries are considered important pathways in the global carbon cycle, carbon dynamics in tropical estuaries is relatively understudied. Here, the tidal, seasonal and spatial variability of particulate organic carbon (POC), dissolved inorganic carbon (DIC), carbon dioxide (CO2) and methane (CH4), among other biogeochemical variables related to carbon cycling, were studied in the Guayas river delta (Ecuador) to document the sources, processing and fluxes of these carbon forms. All variables were studied during a semi-diurnal (13 h) tidal cycle and along river transects at low and high tides, all carried out during one dry and rainy season. POC and total suspended matter (TSM) strongly covaried and peaked at high tidal flow velocities during a tidal cycle and at high river discharge during the rainy season, suggesting that resuspension of bottom sediments and/or surface erosion in the river catchment were a dominant source of particulate matter in the water column. The δ13C of POC, (from ∼-22‰ to ∼-27‰) showed an increasing contribution of marine phytoplankton to the POC pool as moving downstream along the delta during the dry season. Upstream DIC concentrations (∼1200 μmol L−1) were high in the Guayas river delta as compared to other tropical estuarine systems, and the δ13C of DIC revealed a shift from a more phytoplankton dominated source in the dry season and downstream (∼-4‰) to a relatively more terrestrial source in the rainy season and upstream (∼-12.5‰). Both DIC and its δ13C showed slight but consistent deviations from conservative mixing that hint at inputs of 13C depleted DIC from mineralization along the delta. High values of the partial pressure of CO2 (pCO2) observed upstream and in the rainy season (∼5250 μatm), associated with O2 undersaturation (∼60%) and low δ13CDIC, suggest a strongly heterotrophic system, and resulted in high CO2 efflux to the atmosphere. CH4 concentrations were also higher during the rainy than dry season (93.5 ± 62.5 vs. 61.3 ± 39.5 nmol L−1), but unlike pCO2, showed tidal variations similar to TSM and POC, thus alluding to potential CH4 release from sediments during resuspension events at high tidal flow velocities. This explorative survey revealed complex drivers and biogeochemical processes acting upon various spatio-temporal scales which are necessary to consider for a complete understanding of the carbon biogeochemistry in estuarine systems. Similar surveys on estuarine carbon in data scarce regions are encouraged to constrain uncertainties in coastal zone carbon budgets.

Lake water dissolved inorganic carbon dynamics revealed from monthly measurements of radiocarbon in the Fuji Five Lakes, Japan

Lakes are sensitive recorders of anthropogenic activities, as human society often develops in their vicinity. Lake sediments thus have been widely used to reconstruct the history of environmental changes in the past, anthropogenic, or otherwise, and radiocarbon dating provides chronological control of the samples. However, specific values of radiocarbon in different carbon reservoirs due to the different pathways of radiocarbon from the upper atmosphere to the lake, called the radiocarbon reservoir age, is always difficult to evaluate because of dynamic processes in and around lakes. There are few systematic studies on radiocarbon reservoir ages for lakes owing to the complex radiocarbon transfer processes for lakes. Here, we investigate lake waters of the Fuji Five Lakes with monthly monitoring of the radiocarbon reservoir effects. Radiocarbon from dissolved inorganic carbon (DIC) for groundwater and river water is also measured, with resulting concentrations (Δ14C) at their lowest at Lake Kawaguchi in August 2018 (–122.4 ± 3.2‰), and at their highest at Lake Motosu in January 2019 (–22.4 ± 2.5‰), despite a distance of 25 km. However, winter values in both lakes show similar trends of rising Δ14C (about 20‰). Our lake water DIC Δ14C results are compared to previously published records obtained from sediments in Lake Motosu and Lake Kawaguchi. These suggest that total organic carbon and compound-specific radiocarbon found in sediments are heavily influenced by summer blooms of aquatic organisms that fix DIC in water. Thus, future studies to conduct similar analyses at the various lakes would be able to provide further insights into the carbon cycle around inland water, namely understanding the nature of radiocarbon reservoir ages.

Tracing the Carbon Flow in Tropical Watershed Using Stable Isotope Technique

Research on catchment area have traditionally involved concentration and flux measurement to better understand weathering, transport and cycling of materials from land to sea. Potentially, modification of terrestrial environment can alter the carbon flow in a catchment. This research is to characterize dissolved inorganic carbon (DIC) in Sungai Kurau and Tasik Bukit Merah. A progressive depletion of (δ¹³C–DIC: -14.20 ± 0.47‰) towards downstream (δ¹³C–DIC: -24.44 ± 0.59‰) is observed. The trend indicates photosynthesis activity at the upper stream system where microbial respiration process is observed to occur at the Tasik Bukit Merah located at downstream area. The dynamic of carbon pathway is highly affected by allochthonous input and autochthonous process in the catchment system. Land use activities within the catchment can disturb the balance between biological and geological processes which control the carbon pool in Kurau catchment.

Radiocarbon in Dissolved Organic and Inorganic Carbon of the South China Sea

AbstractWe present the carbon isotope (14C and 13C), dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) concentration measurements in the South China Sea (SCS) to reveal the different sources and cycling time scales of the two major carbon pools in the SCS. The DIC concentrations ranged from 1,776 to 2,328 μmol kg−1, and they were lower at the surface and increased with depth. Conversely, the DOC concentrations ranged from 38 to 95 μM, and they were higher on the surface and decreased rapidly in the upper 500‐m water depth. The DIC Δ14C and DOC Δ14C values varied from −227‰ to 68‰ and −557‰ to −258‰, respectively, and both decreased with depth until 1,500 m and then remained relatively constant. DOC Δ14C values were −330‰ lower than DIC Δ14C, indicating that DOC has cycled for much longer than DIC in the SCS. The lower Δ14C‐DIC and Δ14C‐DOC values at depths shallower than 700 m were mainly influenced by intensified vertical mixing, which upwelled the deep water with low Δ14C‐DIC and Δ14C‐DOC values for thorough mixture with the upper layer water. Conversely, the small difference in the Δ14C signature in deep water (>1,500 m) between the SCS and the North Pacific confirmed the rapid water exchange through the Luzon Strait and rapid water mixing in the SCS basin, which plays an important role in controlling carbon cycling in the deep SCS.

Life history of northern Gulf of Mexico Warsaw grouper Hyporthodus nigritus inferred from otolith radiocarbon analysis.

Warsaw grouper, Hyporthodus nigritus, is a western Atlantic Ocean species typically found at depths between 55 and 525 m. It is listed as a species of concern by the U.S. National Marine Fisheries Service and as near threatened by the International Union for the Conservation of Nature. However, little information exists on the species’ life history in the northern Gulf of Mexico (nGOM) and its stock status in that region is currently unknown. Age of nGOM Warsaw grouper was investigated via opaque zone counts in otolith thin sections (max age = 61 y), and then the bomb 14C chronometer was employed to validate the accuracy of age estimates. Otolith cores (n = 14) were analyzed with accelerator mass spectrometry and resulting Δ14C values overlain on a loess regression computed for a regional coral and known-age red snapper Δ14C time series. Residual analysis between predicted Δ14C values from the loess regression versus Warsaw grouper otolith core Δ14C values indicated no significant difference in the two data series. Therefore, the accuracy of otolith-based aging was validated, which enabled growth and longevity estimates to be made for nGOM Warsaw grouper. Dissolved inorganic carbon (DIC) Δ14C values collected from the nGOM support the inference that juvenile Warsaw grouper occur in shelf waters (<200 m) since DIC Δ14C values in this depth range are enriched in 14C and similar to the Δ14C values from otolith cores. A Bayesian model was fit to fishery-dependent age composition data and produced von Bertalanffy growth function parameters of L∞ = 1,533 mm, k = 0.14 y-1, and t0 = 1.82 y. Fishing mortality also was estimated in the model, which resulted in a ratio of fishing to natural mortality of 5.1:1. Overall, study results indicate Warsaw grouper is a long-lived species that is estimated to have experienced significant overfishing in the nGOM, with the age of most landed fish being <10 y.

Dead in the Water: The Vicious Cycle of Blanks During Natural Level 14 C Manipulation of Marine Algal Cultures

Authentic biomarker standards were obtained from algal cultures in an attempt to accurately determine blank C added during sample processing for compound-specific radiocarbon analysis. Emiliania huxleyi and Thalassiosira pseudonana were grown under manipulated Δ14C dissolved inorganic carbon (DIC) levels and chlorophyll a and either alkenones (E. huxleyi) or low molecular weight (LMW) alkanoic acids (T. pseudonana) were isolated from the respective biomass using preparative liquid chromatography (LC), wet chemical techniques or preparative gas chromatography, respectively. DI14C in the seawater medium was determined pre- and post-growth. Biomarker Δ14C values mostly agree within 1-2 analytical uncertainties. In those cases where biomarker Δ14C values differ significantly, chlorophyll a is up to 104‰ more 14C-depleted than alkenones or LMW alkanoic acids, consistent with a larger LC blank compared to the other purification methods. However, in the majority of experimental setups pre- and post-growth DIC Δ14C values seem to be compromised by an unknown and variable blank C contribution. DIC Δ14C values deviate strongly from the anticipated Δ14C values (by up to ca. 560‰), pre- and post-growth Δ14C values differ significantly (by up to ca. 460‰), and changes are not unidirectional. Accordingly, since the substrate Δ14C value cannot unequivocally be constrained, blank C contributions for the different biomarker purification methods cannot be accurately calculated. This study illustrates the challenges and problems of producing authentic standards that are not readily commercially available and exemplifies how a laborious and time-consuming culturing approach may enter a vicious cycle of blank C contamination hampering accurate blank C determination.

Infiltration of Sulfate to Enhance Sulfate‐Reducing Biodegradation of Petroleum Hydrocarbons

AbstractThe lack of sufficient electron acceptors, particularly sulfate, can limit the rate of biodegradation of petroleum hydrocarbons (PHCs). Hence, there is a growing interest by remediation practitioners to deliver sulfate to a PHC impacted saturated zone to enhance biodegradation. When shallow contamination is present in a relatively permeable aquifer and site constraints allow, a cost‐effective approach is to apply sulfate on the ground surface. In this investigation a pilot‐scale experiment was conducted to increase our understanding of the delivery of sulfate using a surface‐based method and the resulting impact on a shallow PHC contaminated aquifer. A surficial infiltration pond positioned on the ground surface above a well‐characterized residual PHC source zone was used to control sulfate dosing. A high‐resolution network near the infiltration pond and downgradient of the source zone was employed to monitor relevant geochemical indicators and PHC concentrations. Compound‐specific isotope analysis (CSIA) was used to identify biodegradation patterns and to investigate the occurrence of microbial sulfate reduction. Selected metabolites and reverse‐transcriptase quantitative polymerase chain reaction analyses of expressed biodegradation genes (as mRNA) were also used to characterize the response of indigenous microorganisms (especially sulfate‐reducing bacteria) to the added sulfate. Three sulfate application episodes (5000 L each) at various Na2SO4 concentrations were allowed to infiltrate under a constant hydraulic head. Although the applied sulfate solution was impacted by density‐driven advection, detailed monitoring data indicated that the sulfate‐enriched water mixed with upgradient groundwater as it migrated downward through the residual PHC zone and formed a co‐mingled downgradient plume with the dissolved PHC compounds. The enrichment of δ34S of sulfate in conjunction with a decrease in sulfate concentration showed the occurrence of sulfate reduction due to the applied sulfate. Increased dissolved inorganic carbon (DIC) concentrations associated with a shift toward more depleted values of δ13C of DIC was indicative of an input of isotopically depleted DIC from biodegradation of PHCs. Despite fluctuations in benzene, toluene, and o‐xylene (BTX) concentrations, the CSIA data for BTX showed that these compounds were biodegraded. The biomarker data provided supporting evidence that toluene and o‐xylene were undergoing anaerobic biodegradation due to sulfate reduction. This study provides insight into factors controlling surface‐based delivery of sulfate to shallow PHC impacted groundwater systems, and the value of isotopic and molecular‐biological procedures to augment conventional monitoring tools.

Quantifying carbon isotope disequilibrium during in-cave evolution of drip water along discreet flow paths

Paleoclimate reconstructions that use speleothem proxy data have increased our understanding of terrestrial climate change, but gaps remain in our understanding of in-cave processes that influence speleothem chemistry. The δ13C values of speleothem calcite are typically influenced by kinetic isotope effects that operate during CO2 degassing and calcite precipitation. Therefore the identification and quantification of these isotopic effects is important in interpreting speleothem stable isotope records. Here we studied the change in water chemistry and δ13C values of dissolved inorganic carbon (DIC) along discreet flow paths at multiple drip sites in Inner Space Cavern, central Texas. We quantified the extent to which the water remains in C isotopic equilibrium during flow along speleothem surfaces as CO2 degasses and calcite precipitates.Two locations in the study cave that have long in-cave flow paths were examined to determine the geochemical evolution and its driving processes along these paths. At each location cave water was sampled at two points 1–2 meters apart along each flow path. Among the key spatial changes observed is a <∼1‰ to ∼4‰ increase in δ13C values of DIC along the flow paths. The magnitude of the increase in δ13C values is controlled by the extent of DIC loss to CO2 degassing. The extent of DIC loss and CO2 degassing is controlled by the pCO2 gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ13C value of the DIC reservoir can be accounted for by a Rayleigh distillation model with equilibrium C-isotope fractionation factors for (CO2(g)-HCO3−(aq)) and (CaCO3-HCO3−(aq)). As the depletion of the DIC reservoir exceeds 15% the DIC δ13C values become progressively higher such that the (HCO3−(aq)-CO2(g)) fractionation values needed to explain the observations change from equilibrium values of ∼8‰ to non-equilibrium values of up to ∼25‰. This variance in magnitude of C-isotope fractionation during CO2 degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss. Such kinetic effects have significant implications for speleothem C-isotope proxy interpretations, as these kinetic isotope effects are of a similar magnitude as those used to infer past changes in drought and vegetation.

Submarine groundwater discharge drives biogeochemistry in two Hawaiian reefs

AbstractGroundwater inputs are typically overlooked as drivers of environmental change in coastal reef studies. To assess the impact of groundwater discharge on reef biogeochemistry, we examined two fringing reef environments, located in Maunalua Bay on the south shore of O‘ahu, Hawai‘i, that receive large inputs of submarine groundwater discharge. We supplemented 25‐ and 30‐d time series measurements of salinity, water temperature, pH, dissolved oxygen, and 222Rn with high‐resolution 24‐h nutrient, dissolved inorganic carbon (DIC), total alkalinity (TA), and δ13C–DIC measurements to evaluate both groundwater‐induced and biologically‐driven changes in coastal carbonate chemistry across salinity gradients. Submarine groundwater discharge at these two locations was characterized by low pHT (7.36–7.62), and variable DIC (1734–3046 μM) and TA (1716–2958 μM) content relative to ambient seawater. Groundwater‐driven variability in coastal carbonate system parameters was generally on the same order of magnitude as biologically‐driven variability in carbonate system parameters at our study locations. Further, our data revealed a shift in reef metabolism from net dissolution to net calcification across this groundwater‐driven physicochemical gradient. At sites with high levels of groundwater exposure, net community production and calcification rates were reduced. Our findings shed light on the importance of considering groundwater inputs when examining coastal carbonate chemistry.

Watershed geomorphology interacts with precipitation to influence the magnitude and source of CO2 emissions from Alaskan streams

AbstractBoreal ecosystems contain a large fraction of the world's soil carbon and are warming rapidly, prompting efforts to understand the role of freshwaters in carbon export from these regions. We examined geomorphic controls on the magnitude of stream CO2 emissions and sources of stream dissolved inorganic carbon (DIC) across a heterogeneous river basin in SW Alaska. We found that watershed slope and precipitation interact to control gaseous carbon fluxes from streams, with the highest flux from low‐gradient watersheds following rainstorms. Watershed slope influences C loading and stream CO2 fluxes by controlling carbon accumulation in watersheds and to a lesser extent by determining gas transfer velocity across the stream air‐water interface. Low gas transfer velocity in flat streams offsets some of the effects of higher terrestrial C loading at those sites, resulting in lower than expected vertical CO2 fluxes. While the isotopic composition of stream DIC (Δ14C and δ13C) was highly variable across space and time, shifts toward contemporary, terrestrial sources after precipitation were most pronounced in flat watersheds. At base flow DIC was a mixture of modern and aged sources of biogenic and geologic origin (Δ14C, −198.3‰ to 27.9‰, n = 23). Aged (Δ14C‐depleted) C sources contributed to food webs via a pathway from DIC to algae to invertebrate grazers. We observed coherent changes in stream carbon chemistry after large rainstorms despite considerable physical heterogeneity among watersheds. These patterns provide a way to extrapolate across boreal landscapes by constraining CO2 concentration and flux estimates by local geomorphic features.

Modeling 400–500-kyr Pleistocene carbon isotope cyclicity through variations in the dissolved organic carbon pool

The carbon isotope (δ13C) record from the Plio-Pleistocene shows prominent 400-kyr cycles with maximum values at eccentricity minima during the Pliocene. The period extends to 500kyr in the Pleistocene after ~1.6Ma. Five δ13C maxima occurred at ~0.2, 0.5, 1.0, 1.5 and 1.9Ma over the last 2Ma. Although several hypotheses have been suggested to explain why the 400–500-kyr cycles are so strong in δ13C records and how they may have originated, the mechanism is still not clear. The aim of this study was to test the dissolved organic carbon (DOC) hypothesis, which was proposed recently to explain this 400–500-kyr cycle in deeper time. We used an intermediate complexity box model that is computationally efficient for studies involving longer timescales. The model incorporates sophisticated microbial processes, dividing the oceanic carbon cycle into a rapid and a slow cycle. The model result suggests that when more nutrients enter the surface ocean, the rapid carbon cycle is more active, and less refractory DOC (RDOC) is produced. The opposite sequence occurs when fewer nutrients enter the ocean. The modeled RDOC concentration and the δ13C of dissolved inorganic carbon (DIC) are anti-correlated with riverine nutrient input. According to mass conservation, the release of isotopically lighter carbon from the RDOC pool leads to lighter DIC δ13C while an increase in the RDOC pool enriches it. The transient simulations produced a one-to-one correspondence between modeled and measured δ13C. This study supports the hypothesis that chemical weathering-induced variations in the DOC pool act as a pacemaker for δ13C changes over 400–500-kyr cycles.

MoDIE: Moderate dissolved inorganic carbon (DI13C) isotope enrichment for improved evaluation of DIC photochemical production in natural waters

Photochemical reactions in natural waters are a sink for dissolved organic carbon (DOC) and a source of dissolved inorganic carbon (DIC). Although considered significant to the carbon budget of the oceans, DIC photoproduction rates remain poorly constrained due to the analytical challenge involved in accurately measuring very low production rates (likely sub-μM h− 1 for blue ocean waters) relative to high background DIC concentrations (~ 2 mM). In an attempt to overcome this analytical limitation, almost all previous DIC photoproduction studies in marine systems have relied on the stripping of background DIC prior to irradiation, with unknown consequences for the integrity of the DOC pool and its photoreactivity, and none have resulted in a satisfactory determination of photoproduced DIC in open ocean waters. Here, we use small additions of NaH13CO3 (< 10% of background DIC concentrations) to achieve moderate DI13C isotope enrichment (MoDIE) of river, estuarine, and seawater samples. We then quantify the shift in δ13C of DIC in irradiated samples via liquid chromatography - isotope ratio mass spectrometry (LC-IRMS) and use these shifts to calculate photoproduced DIC. MoDIE provides a new method to more precisely (± 56 to ± 340 nM DIC) and more rapidly quantify DIC photoproduction compared to previous methods. MoDIE was evaluated by determining initial DIC photoproduction rates, along with associated broadband photochemical efficiency data (ranging from 144 to 280 μmol DIC per mol photons absorbed), in riverine to offshore waters, and produced the earliest time point and most precise measurement of DIC photoproduction in unmodified, low-CDOM, blue water (ag(325) = 0.30 m− 1) reported to date. In addition to measuring small DIC changes from photochemistry, the use of MoDIE could provide a major advance for other ocean carbon cycling studies, including examination of oceanic respiration, carbonate mineral precipitation/dissolution, and assessment of ocean acidification.

Technical note: Coupling infrared gas analysis and cavity ring down spectroscopy for autonomous, high-temporal-resolution measurements of DIC and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C–DIC

Abstract. A new approach to autonomously determine concentrations of dissolved inorganic carbon (DIC) and its carbon stable isotope ratio (δ13C–DIC) at high temporal resolution is presented. The simple method requires no customised design. Instead it uses two commercially available instruments currently used in aquatic carbon research. An inorganic carbon analyser utilising non-dispersive infrared detection (NDIR) is coupled to a Cavity Ring-down Spectrometer (CRDS) to determine DIC and δ13C–DIC based on the liberated CO2 from acidified aliquots of water. Using a small sample volume of 2 mL, the precision and accuracy of the new method was comparable to standard isotope ratio mass spectrometry (IRMS) methods. The system achieved a sampling resolution of 16 min, with a DIC precision of ±1.5 to 2 µmol kg−1 and δ13C–DIC precision of ±0.14 ‰ for concentrations spanning 1000 to 3600 µmol kg−1. Accuracy of 0.1 ± 0.06 ‰ for δ13C–DIC based on DIC concentrations ranging from 2000 to 2230 µmol kg−1 was achieved during a laboratory-based algal bloom experiment. The high precision data that can be autonomously obtained by the system should enable complex carbonate system questions to be explored in aquatic sciences using high-temporal-resolution observations.

Inorganic carbon transformations between phases and its impact on its isotopic signature under open conditions

In order to evaluate the possibility of tracing dissolved inorganic carbon (DIC) derived from calcite dissolution under open system conditions, controlled laboratory experiments using stable carbon isotopes were conducted in a specially designed reaction chamber with constant pH monitoring. The system design also allowed us to measure the forward and backward (degassing) gas transfer reactions rates. We confirmed that the degassing reaction rate can be two to four times slower than the gas transfer reaction rate depending on the differences in pCO2 between the gas phase and the liquid. The isotopic analyses suggest that the carbon isotopic signature of DIC under open system conditions is controlled by the CO2 carbon isotopic signature, the isotopic fractionation between CO2, the relative occurrence of the different DIC species, and changes in pH. We found that the isotopic signature of DIC is not affected by the isotopic signature of the calcite or the mechanism controlling the dissolution (protonation or hydration). Practically this study suggests that it is not possible to trace the DIC derived from calcite dissolution under open system conditions. It supports the approach of evaluating the pedogenic carbonate content by stable carbon isotopes methods. In open system conditions: a) pH can explain up to 8‰ variation in the DIC δ13C signature and b) temperature is the second most important factor affecting the δC13-DIC signature, and only at pH above neutrality.