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The transport of carbon from land to ocean, via rivers, groundwater, and aerosols, is an important component of the global carbon cycle that must be known to accurately assess anthropogenic CO2 storage on land and in the ocean. Current global carbon cycle budgets have adopted terrestrial carbon inputs to the ocean ranging from 0.5 GtC/yr to 0.9 GtC/yr, derived mainly from estimates of riverine fluxes. However, these budgets ignore the terrestrial carbon inputs from coastal ecosystems and through submarine groundwater discharge (collectively referred to as coastal margin inputs in this study) due to difficulties in making global assessments. Using a numerical model and globally distributed ocean observations of stable carbon isotopes, we estimate terrestrial carbon inputs to the ocean at 1.4 +/- 0.5 GtC/yr, with 95% of the coastal margin inputs (0.8 +/- 0.5 GtC/yr) occurring in the Pacific and Indian Oceans. This terrestrial carbon flux is largely balanced by an efflux to the atmosphere of 1.2 +/- 0.5 GtC/yr, 40% of which occurs in poorly monitored coastal regions and may have been overlooked in previous observation-based global estimates. Our results suggest more dynamic cycling of carbon in the land-ocean transition zone than previously thought, and that rivers may not be the only important pathway for terrestrial carbon to the ocean.

The distribution of fluorescent dissolved organic matter (FDOM) in the East Sea (Sea of Japan) was examined by excitation–emission matrix spectroscopy (EEMs) with parallel factor analysis (PARAFAC). Humic-like FDOM (FDOMH) increased with depth and was significantly correlated with Apparent Oxygen Utilization (AOU), indicating that FDOMH in the deep water is mainly produced by oxidation of organic matter. In addition, a surprisingly large excess of FDOMH relative to that expected from the observed AOU was found from 1000 m to the bottom (up to 3500 m). Based on the high-resolution geographical distribution and characteristics of FDOM in the East Sea, we conclude that this excess likely originates from anaerobic FDOMH production in subsurface bottom sediments. This FDOMH flux accounts for 8–15% of the total FDOM production in the water column. Our results suggest that anaerobic activities in subsurface sediments are an important hidden source of FDOM in the ocean.

Radium isotopes (228Ra and 226Ra) are excellent tracers of submarine groundwater discharge (SGD). To estimate SGD magnitudes, information on the end-member values of Ra concentrations in groundwater is critical; however, the distribution characteristics of Ra in coastal aquifers are poorly understood. In this study, we show that Ra concentrations in coastal groundwater are primarily dependent on salinity based on the data (n > 500) obtained from global coastal aquifers, although previous end-member calculations averaged all Ra concentrations without considering salinity. If we assume that SGD is composed mainly of seawater infiltrating the aquifer, previous estimates of SGD for the Atlantic Ocean and the global ocean were overestimated twofold to threefold. This may be similar for other applications using different Ra isotopes. Our study highlights that the end-members of Ra isotopes in groundwater should be carefully considered when estimating SGD using Ra isotope mass balances in the ocean.

Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013 m3 yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.

We measured Ra isotopes (223Ra and 228Ra) in surface seawater of the northwestern Pacific marginal seas to trace the flow rate and mixing of the Changjiang diluted water (CDW) in the summer of 2012. Based on the horizontal distribution of 223Ra activities, the arrival time of CDW from the river mouth to 450 km offshore northeast was estimated to be 20–35 days, which is similar to that determined in previous studies. Moreover, we successfully calculated the relative contribution of CDW at each sampling station using a salinity and 228Ra diagram. Using this unique method, we found that the relative contribution of CDW was more than 30% in most surface seawaters of the northern East China Sea, the Yellow Sea, and the southern sea off Korea. Our results suggest that CDW is of high significance in the biogeochemistry of surface seawater of these northwestern Pacific marginal seas during the summer monsoon period.

The naturally occurring radionuclide tracers, 234Th-238U and 210Po-210Pb pairs, were measured bimonthly at the Bermuda Atlantic Time-Series Study (BATS) site in the Sargasso Sea from October 1996 to August 1997. Biological production and biogeochemical fluxes of thorium, polonium, and particulate organic carbon were highest in December 1996 and August 1997. Although the enhanced production and fluxes in December 1996 could be due to winter nitrate inputs from the subsurface ocean, those in August 1997 were not expected, on the basis of the nutrient mass balance and extreme stratification of the Sargasso Sea during this summer. The fixed-nutrient inputs from the atmosphere, eddy intrusion, or deepwater upwelling were unlikely sources of such elevated summer productivity. Alternatively, the unusually high atmospheric “wet” deposition of Fe, which occurred during this summer period, appears to have fueled high nitrogen fixation and thus greater carbon and reactive-element export. Also as important, ocean production did not respond to significant “dry” deposition of Fe. This seems to be due to the reported lower solubility of Fe dust in seawater versus rainwater.

Atmospheric wet and bulk depositional fluxes of trace elements (Fe, Al, Mn, Zn, Pb, Cu, Cd, Ni, and Cr), 210Pb, and 7Be (bulk only) to the Sargasso Sea were measured at Bermuda from September 1996 to September 1997. Wet deposition was the dominant process of trace element deposition over most of the sampling seasons, based on the measured bulk versus wet depositional fluxes. Although about half of the Mn in Bermuda precipitation was calculated to originate from noncrustal sources, its seasonal trend is similar to the Fe and Al, suggesting primary sources from continental crustal materials. On the other hand, Cd, Zn, Pb, Cu, Ni, and Cr in Bermuda precipitation originate mainly from noncrustal sources (presumably anthropogenic) based on enrichment factor analyses. The noncrustal sources were highest during the fall-winter due to more frequent intrusion of U.S. continental air as the Bermuda High weakens. Atmospheric depositional fluxes of 210Pb showed a good correlation with the 7Be fluxes (r2=0.84) at Bermuda, indicating major transport of 210Pb via the upper troposphere from continents, along with other trace elements. Annual depositional fluxes of noncrustal elements (Cd, Zn, Pb, Cu, Ni, and Cr) at Bermuda were a factor of 2 to 3 lower than those at the mid-Atlantic Bight and were about 2–20 fold lower during the sampling year than those during the early 1980s. This confirms the rapid evolution of trace element inputs to the upper Sargasso Sea during the last few decades resulting from industrial emission controls in the surrounding continents.

[1] The magnitude of submarine fresh groundwater discharge (SFGD) was estimated using a freshwater budget of the coastal water, which was calculated from salinity anomalies off a volcanic island, Jeju, Korea. We obtained high-resolution profiles of salinity from the entire coastal area of Jeju Island using accurate CTD sensors. Lower salinity anomalies relative to the pristine offshore seawaters were observed and assumed to be a result of SFGD on the basis of negligible surface runoffs from the island and the relatively high levels of 222Rn in the low-salinity waters. The SFGD flux during the study period calculated from a freshwater budget of the water column was 2.5−3.8 × 106 m3 d−1, which is close to the long-term average based on the inland water balance. Our study shows that SFGD can be accurately and easily measured using a freshwater budget of the coastal waters off a highly permeable coastal zone with little surface runoff.

[1] In order to determine the physical and chemical factors controlling the outbreak of red tides, we monitored nutrients and other environmental parameters using a novel real-time monitoring buoy system during the summer of 2003 in the southern sea of Korea, where red-tide outbreaks occur every year. The real-time monitoring data on bioluminescence, which may indicate the presence of bioluminescent dinoflagellate species, showed a sudden increase under the lowest concentrations of dissolved inorganic nutrients. Our monitoring system for the first time provides real-time variations in nutrients and associated outbreaks of dinoflagellate red tides. This result supports the previous hypothesis by others that the outbreak of dinoflagellate red tides is associated with the limited growth of diatoms under depleted DIP or DIN conditions. We suggest that this real-time monitoring system can be utilized as a powerful tool for studying and predicting harmful dinoflagellate red tides in the coastal ocean.

[1] During a 3-year record of 228Ra/226Ra ratio profiles (from nine cruises) in the Sargasso Sea, two profiles in the Spring (April 1997 and 1998) show distinct subsurface maxima. Such maxima in the radium isotope ratio record the subsurface advection of surface waters, such as subtropical mode waters. The April 1997 maximum occurred below the euphotic zone (between 300–400 m) and was also coincident with that of POC and dissolved oxygen, supporting the subsurface advection of surface waters. This single event represents a large isopycnal injection (23 g m−2) of organic carbon into the subsurface ocean. Although we cannot quantify carbon export by this process on a basin scale based on these single-site measurements, it may be an important pathway for short-term oceanic carbon export.

[1] We measured seepage rates of coastal groundwater from a volcanic island, standing in the South Sea of Korea. The seepage rates measured along the sandy coast were in the range 50–300 m/yr, which are much higher than those reported from typical continental coast. The 18O tracer in potential seeping waters signifies that they are mixtures of basal groundwater and pristine seawaters. On the eastern coast of Jeju, almost all groundwater discharge is attributed to recirculating seawater, while fresh groundwater contributes about 20% of the total submarine groundwater discharge (SGD) on the western coast of Jeju. Thus, SGD appears to be important for the management of water resources in western Jeju. The measured radionuclides (228Ra, 226Ra, and 222Rn) and nutrients in the groundwater suggest that the discharge of both fresh and recirculated seawater will have a significant influence on the budget of coastal nutrients and other chemical constituents in this region.

[1] Atmospheric samples were collected aboard ship in the South and equatorial Atlantic (35°S–10°N) between 19 May and 20 June 1996. We measured 222Rn in air, 210Pb in aerosol, and trace elements (Fe, Mn, Zn, Pb, Cu, Cd, Ni, and Cr), 210Pb, and 210Po in precipitation samples. The large variation of 222Rn in air suggests a significant change in the incursion of continental air with time and latitude in the remote Atlantic. In the equatorial and subtropical Atlantic (20°S–10°N), 222Rn activity was lower but 210Pb/222Rn ratios were higher than those at higher latitudes. The higher 210Pb/222Rn ratios in the equatorial Atlantic appear to be due to prevailing trade easterly winds which transport a supported source of 210Pb in Saharan dust from the African Sahel. The enrichment of noncrustal trace elements in precipitation samples from the remote equatorial Atlantic was small on account of the remoteness from the continental emission regions and as a result of dilution with Saharan dust. The wet depositional fluxes of major crustal elements (Fe and Mn) were two- to three-fold higher, while those of Cd and Zn were two- to ten-fold lower, in the South and equatorial Atlantic relative to the western North Atlantic (Bermuda) or North Atlantic coast (Lewes, Delaware). Thus, dominant wet precipitation of Saharan dust in the Intertropical Convergence Zone (ITCZ) areas of the equatorial Atlantic appears to be a large potential source of micronutrients (i.e., Fe) to surface seawater.

[1] There has emerged a recognition that the submarine discharge of fresh, brackish, and marine groundwaters into the coastal ocean is comparable to the inputs via river discharge. However, the factors controlling submarine groundwater discharge (SGD) are poorly understood owing to a lack of SGD measurement tools in dynamic coastal waters. We observed bi-hourly variations of the natural SGD tracers, 222Rn and CH4, at a coastal seawater station over two seasons using novel monitoring techniques. This unique data set suggests that SGD increases sharply from neap to spring tide during the wet season. The observed increase is much greater than what would be expected from the rainfall and tidal-height changes. We conclude from this that the temporal variation of SGD is regulated predominantly by a semi-monthly fluctuation of a tidal oscillating pumping force in this environment.

[1] We observed a sudden initiation of bottom-water formation in the East/Japan Sea associated with a severely cold winter in 2000–2001. An increase in dissolved oxygen concentration as well as decreases in temperature and nutrient concentrations for the bottom waters provides unequivocal evidence that cold, oxygen-rich and nutrient-poor surface waters were injected directly to the bottom. Since the conveyor-belt in the East Sea has been undergoing dramatic change with a complete halt to bottom-water formation since the mid-1980s, this sudden episode of bottom-water formation could easily be detected. Though the amount of bottom water formed was rather small, being only about 0.03% of the volume in the past time, the observation clearly demonstrates that the conveyor-belt is directly connected to the weather system.