Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation

We present the first set of dissolved silicon isotope data in seawater (delta Si-30(Si(OH)4)) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface delta Si-30(Si(OH)4) signatures of +2.1 parts per thousand corresponding to the highest Si(OH)(4) concentrations (similar to 30.0 mu mol L-1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface delta Si-30(Si(OH)4) increased rapidly to +3.2 parts per thousand while Si(OH)(4) became depleted, indicating increasing biological utilization of the Si(OH)(4) originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of delta Si-30(Si(OH)4) and an increase of Si(OH)(4) with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the delta Si-30(Si(OH)4) was nearly constant at +1.9 parts per thousand throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in delta Si-30(Si(OH)4) were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were similar to 1.0 parts per thousand lower than those in summer, despite the same primary Si(OH)(4) supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)(4) in summer than in winter

Coccolithophore responses to environmental variability in the South China Sea: species composition and calcite content

Coccolithophore contributions to the global marine carbon cycle are regulated by the calcite content of their scales (coccoliths) and the relative cellular levels of photosynthesis and calcification rates. All three of these factors vary between coccolithophore species and with response to the growth environment. Here, water samples were collected in the northern basin of the South China Sea (SCS) during summer 2014 in order to examine how environmental variability influenced species composition and cellular levels of calcite content. Average coccolithophore abundance and their calcite concentration in the water column were 11.82 cells mL−1 and 1508.3 pg C mL−1, respectively, during the cruise. Water samples can be divided into three floral groups according to their distinct coccolithophore communities. The vertical structure of the coccolithophore community in the water column was controlled by the trophic conditions, which were regulated by mesoscale eddies across the SCS basin. The evaluation of coccolithophore-based calcite in the surface ocean also showed that three key species in the SCS (Emiliania huxleyi, Gephyrocapsa oceanica, Florisphaera profunda) and other larger, numerically rare species made almost equal contributions to total coccolith-based calcite in the water column. For Emiliania huxleyi biometry measurements, coccolith size positively correlated with nutrients (nitrate, phosphate), and it is suggested that coccolith length is influenced by light and nutrients through the regulation of growth rates. Larger-sized coccoliths were also linked statistically to low pH and calcite saturation states; however, it is not a simple cause and effect relationship, as carbonate chemistry was strongly co-correlated with the other key environmental factors (nutrients, light)

Constraining the oceanic barium cycle with stable barium isotopes

Highlights • We present a Ba isotope data set of seawater, river waters and biogenic particles. • Ba isotope signatures of upper ocean waters are heavier than river and deep waters. • Adsorption of lighter Ba isotopes on biogenic particles induces the fractionation. • Ba isotopes trace land–sea interactions and ocean mixing processes. • Decoupling of Ba from major nutrients confirms Ba to be a biointermediate element. Abstract The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope (δ137BaDBa) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang (Yangtze River). We find pronounced and systematic horizontal and depth dependent δ137BaDBa gradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3‰+0.3‰), the δ137BaDBa values in the upper water column are significantly higher (+0.9‰+0.9‰) than those in the deep waters (+0.5‰+0.5‰). Moreover, δ137BaDBa signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and δ137BaDBa from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The subsurface δ137BaDBa distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land–sea interactions and ocean mixing processes

Sources and accumulation of organic carbon in the Pearl River Estuary surface sediment as indicated by elemental, stable carbon isotopic, and carbohydrate compositions

© The Authors, 2010. This article is distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Biogeosciences 7 (2010): 3343-3362, doi:10.5194/bg-7-3343-2010.Organic matter in surface sediments from the upper reach of the Pearl River Estuary and Lingdingyang Bay, as well as the adjacent northern South China Sea shelf was characterized using a variety of techniques, including elemental (C and N) ratio, bulk stable organic carbon isotopic composition (δ13C), and carbohydrate composition analyses. Total organic carbon (TOC) content was 1.21±0.45% in the upper reach, down to 1.00±0.22% in Lingdingyang Bay and to 0.80±0.10% on the inner shelf and 0.58±0.06% on the outer shelf. δ13C values ranged from −25.1‰ to −21.3‰ in Lingdingyang Bay and the South China Sea shelf, with a trend of enrichment seawards. The spatial trend in C/N ratios mirrored that of δ13C, with a substantial decrease in C/N ratio offshore. Total carbohydrate yields ranged from 22.1 to 26.7 mg (100 mg OC)−1, and typically followed TOC concentrations in the estuarine and shelf sediments. Total neutral sugars, as detected by the nine major monosaccharides (lyxose, rhamnose, ribose, arabinose, fucose, xylose, galactose, mannose, and glucose), were between 4.0 and 18.6 mg (100 mg OC)−1 in the same sediments, suggesting that significant amounts of carbohydrates were not neutral aldoses. Using a two end-member mixing model based on δ13C values and C/N ratios, we estimated that the terrestrial organic carbon contribution to the surface sediment TOC was ca. 78±11% for Lingdingyang Bay, 34±4% for the inner shelf, and 5.5±1% for the outer shelf. The molecular composition of the carbohydrate in the surface sediments also suggested that the inner estuary was rich in terrestrially derived carbohydrates but that their contribution decreased offshore. A relatively high abundance of deoxyhexoses in the estuary and shelf indicated a considerable bacterial source of these carbohydrates, implying that sediment organic matter had undergone extensive degradation and/or transformation during transport. Sediment budget based on calculated regional accumulation rates showed that only ~50% of the influxes of terrestrial organic carbon were accumulated in the estuary. This relatively low accumulation efficiency of terrestrial organic matter as compared to the total suspended solids (accumulation efficiency ~73%) suggested significant degradation of the terrestrial organic carbon within the estuarine system after its discharge from the river. This study demonstrated that the combination of the bulk organic matter properties together with the isotopic composition and molecular-level carbohydrate compositions can be an efficient way to track down the source and fate of organic matter in highly dynamic estuarine and coastal systems. The predominance of terrestrially originated organic matter in the sediment and its generally low accumulation efficiency within the estuary is not surprising, and yet it may have important implications in light of the heavy anthropogenic discharges into the Pearl River Estuary during the past thirty years.This research was supported by the Natural Science Foundation of China through grants #49825111, #40176025 and #90211020

The bioavailability of riverine dissolved organic matter in coastal marine waters of southern Texas

Abstract(#br)To examine the bioavailability of dissolved organic carbon (DOC) and nitrogen (DON) in riverine dissolved organic matter (DOM) discharged to the coastal ocean, we conducted a series of month-long (24 days) incubation experiments with filtered samples collected from five southern Texas rivers (Lavaca, San Antonio, Mission, Aransas, and Nueces) inoculated using the same natural coastal microbial assemblages during summer (June) and winter (January) in 2016. The bioavailable fractions of DOC and DON (BDOC% and BDON%) varied substantially in different rivers and seasons, ranging respectively from 6 to 11%, and 15–38% during winter, and 0–6% and 9–15% during summer. Relatively higher BDOC% and BDON% occurred in the San Antonio and Aransas Rivers, which are impacted more by human activities through discharge from wastewater treatment plants. Seasonally, the riverine DOM was more bioavailable in winter than in summer when DOM may have been extensively degraded in situ due to the low base flow (or long residence time) and the elevated temperature in river water in summer. The principal component analysis on amino acid composition further confirmed that DOM was less degraded in winter than in summer. Functional gene abundance data revealed that winter riverine DOM was relatively labile as evidenced by an increase in N-metabolism pathways and functional genes during the winter incubation, whereas the opposite pattern was observed in summer. The findings of the varying bioavailability of DOM among rivers and seasons have important implications about the fate of riverine DOM and their potential contributions to nutrient supplies as southern Texas bays and estuaries are often nitrogen limited

On the seasonal variation of air-sea CO2 fluxes in the outer Changjiang (Yangtze River) Estuary, East China Sea

Based upon seven field surveys conducted during April 2005 - April 2008, we examined the surface partial pressure of CO2 (pCO(2)) and dissolved oxygen (DO) in the outer Changjiang (Yangtze River) Estuary, on the inner shelf of the East China Sea (ECS). This area represents a most dynamic zone of the ECS where high pCO(2) riverine water meets with highly productive shelf waters, covering a 2 degrees x 3 degrees area, similar to 10% of the surface area of the entire ECS. Surface pCO(2) ranged 320 - 380 mu atm (average similar to 345 mu atm) in winter, 180 - 450 mu atm (average similar to 330 mu atm) in spring, 150 - 620 mu atm (average similar to 310 mu atm) in summer and 120 - 540 mu atm (average similar to 375 mu atm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO(2), ranging 95% - 105% in winter, 96% - 142% (average 110%) in spring, 73% - 192% (average 718%) in summer and 81% - 178% (average 102%) in autumn. The dynamics of pCO(2) drawdown and DO enhancement in the warm seasons (from April to October) appeared to be controlled by primary productivity and air - sea exchange, while mixing dominated the aqueous pCO(2) in the cold seasons (from November to March of the following year). This study showed that the outer Changjiang Estuary served as a moderate or significant sink of atmospheric CO2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea - air CO2 flux in the outer Changjiang Estuary was estimated as -1.9 +/- 1.3 mol m(-2) year(-1), which is double the recent sea-air CO2 flux estimation for the northern ECS. (C) 2009 Elsevier B.V. All rights reserved

How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 9 (2012): 1777-1795, doi:10.5194/bg-9-1777-2012.In order to assess the role of submarine groundwater discharge (SGD) and its impact on the carbonate system on the northern South China Sea (NSCS) shelf, we measured seawater concentrations of four radium isotopes 223,224,226,228Ra along with carbonate system parameters in June–July, 2008. Complementary groundwater sampling was conducted in coastal areas in December 2008 and October 2010 to constrain the groundwater end-members. The distribution of Ra isotopes in the NSCS was largely controlled by the Pearl River plume and coastal upwelling. Long-lived Ra isotopes (228Ra and 226Ra) were enriched in the river plume but low in the offshore surface water and subsurface water/upwelling zone. In contrast, short-lived Ra isotopes (224Ra and 223Ra) were elevated in the subsurface water/upwelling zone as well as in the river plume but depleted in the offshore surface water. In order to quantify SGD, we adopted two independent mathematical approaches. Using a three end-member mixing model with total alkalinity (TAlk) and Ra isotopes, we derived a SGD flux into the NSCS shelf of 2.3–3.7 × 108 m3 day−1. Our second approach involved a simple mass balance of 228Ra and 226Ra and resulted in a first order but consistent SGD flux estimate of 2.2–3.7 × 108 m3 day−1. These fluxes were equivalent to 12–21 % of the Pearl River discharge, but the source of the SGD was mostly recirculated seawater. Despite the relatively small SGD volume flow compared to the river, the associated material fluxes were substantial given their elevated concentrations of dissolved inorganic solutes. In this case, dissolved inorganic carbon (DIC) flux through SGD was 153–347 × 109 mol yr−1, or ~23–53 % of the riverine DIC export flux. Our estimates of the groundwater-derived phosphate flux ranged 3–68 × 107 mol yr−1, which may be responsible for new production on the shelf up to 0.3–6.3 mmol C m−2 d−1. This rate of new production would at most consume 11 % of the DIC contribution delivered by SGD. Hence, SGD may play an important role in the carbon balance over the NSCS shelf.This work was financially supported by the National Basic Research Program of China (973 Program) through grant #2009CB421204 and #2009CB421201, and by the Natural Science Foundation of China (NSFC) through grants #90711005, #41121091 and #41130857. Matthew Charette’s participation was supported by a grant from the U.S. National Science Foundation (#OCE-0751525)