Annual Lateral Organic Carbon Exchange Between Salt Marsh and Adjacent Water: A Case Study of East Headland Marshes at the Yangtze Estuary

Blue carbon (C) ecosystems (mangroves, salt marshes, and seagrass beds) sequester high amounts of C, which can be respired back into the atmosphere, buried for long periods, or exported to adjacent ecosystems by tides. The lateral exchange of C between a salt marsh and adjacent water is a key factor that determines whether a salt marsh is a C source (i.e., outwelling) or sink in an estuary. We measured salinity, particulate organic carbon (POC), and dissolved organic carbon (DOC) seasonally over eight tidal cycles in a tidal creek at the Chongming Dongtan wetland from July 2017 to April 2018 to determine whether the marsh was a source or sink for estuarine C. POC and DOC fluxes were significantly correlated in the four seasons driven by water fluxes, but the concentration of DOC and POC were positively correlated only in autumn and winter. DOC and POC concentrations were the highest in autumn (3.54 mg/L and 4.19 mg/L, respectively) and the lowest in winter and spring (1.87 mg/L and 1.51 mg/L, respectively). The tidal creek system in different seasons showed organic carbon (OC) export, and the organic carbon fluxes during tidal cycles ranged from –12.65 to 4.04 g C/m2. The intensity showed significant seasonal differences, with the highest in summer, the second in autumn, and the lowest in spring. In different seasons, organic carbon fluxes during spring tides were significantly higher than that during neap tides. Due to the tidal asymmetry of the Yangtze River estuary and the relatively young stage, the salt marshes in the study area acted as a strong lateral carbon source.

Permafrost thaw induced abrupt changes in hydrology and carbon cycling in Lake Wudalianchi, northeastern China

Lakes in the permafrost zone have been proposed to serve as key outlets for methane and carbon dioxide emissions. However, there has been no geological record of the hydrological and biogeochemical responses of lakes throughout the thawing of surrounding permafrost. We use multiple biomarker and isotopic proxies to reconstruct hydrological and biogeochemical changes in Lake Wudalianchi in northeastern China during regional thawing of the permafrost. We show permafrost thawing, as indicated by lignin degradation, initiated rapid lake water freshening as a result of the opening of groundwater conduits, and negative organic δ13C excursion due to increased inorganic and organic carbon fluxes. These hydrological changes were followed, with an ~5–7 yr delay, by abrupt and persistent increases in microbial lake methanotrophy and methanogenesis, indicating enhanced anaerobic organic decomposition and methane emissions from lakes as permafrost thaws. Our data provide a detailed assessment of the processes involved during permafrost thaw, and highlight the importance of lakes in ventilating greenhouse gases to the atmosphere.

Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran–Phanerozoic Marine Mudrocks

Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments <500 μg/g, but many ancient marine organic-rich mudrocks record enrichments >500 μg/g. Previous studies propose that ancient V enrichments of these magnitudes (“V hyper-enrichments”) were deposited from hyper-sulfidic bottom-waters with higher H2S levels (≥10 mM) than observed in modern euxinic basins. To test the importance of hyper-sulfidic conditions for generating V hyper-enrichments, we compare V concentrations with Mo isotope (δ98Mo) compositions from mudrock samples ranging in age from Ediacaran to Pleistocene. In the modern ocean, sediments deposited from strongly euxinic bottom waters ([H2S]aq > 11 μM) closely record global seawater δ98Mo because conversion of molybdate to tri- and tetra-thiomolybdate is quantitative. By contrast, large Mo isotope fractionations occur during Mo adsorption to Fe-Mn particulates or because of incomplete formation of the most sulfidic thiomolybdates in weakly euxinic settings ([H2S]aq < 11 μM), which both favor removal of lighter-mass Mo isotopes to sediments. We find multiple examples when mudrocks with V hyper-enrichments are associated with a wide range of δ98Mo for a single time interval, including values at or below oceanic input δ98Mo (0.3–0.7‰). This observation suggests significant isotopic offset from reasonable seawater values (typically ≥1.0‰). Thus, we conclude that hyper-sulfidic conditions were not responsible for many V hyper-enrichments in Ediacaran–Phanerozoic mudrocks. Instead, sediment V hyper-enrichments can be explained by high Fe-Mn particulate fluxes to weakly euxinic sediments or by moderately restricted euxinic settings with strongly euxinic ([H2S]aq > 11 μM but not necessarily > 10 mM) or weakly euxinic (with slow clastic sedimentation rates and high organic carbon fluxes) bottom waters where vigorous water exchange provides a continuous V supply from the open ocean.

Sub-annual fluorescence measurements of coral skeleton: relationship between skeletal luminescence and terrestrial humic-like substances

Some massive coral core slices reveal luminescent bands under ultraviolet light, which have been attributed to terrestrial humic acids in the skeleton. Coral luminescence has therefore been used to reconstruct past climate and hydrological variability. However, it has remained unresolved how closely coral luminescence at sub-annual resolution is related to terrestrial humic acid concentrations. This study presents a solution-based fluorescence method to quantify terrestrial humic substances in less than 4 mg of coral powder. The results show that in corals from Malaysia and Singapore, the luminescence green-to-blue ratio is correlated with skeletal concentrations of terrestrial humic substances (R2 > 0.40, p < 0.001) at two sites that are exposed to terrestrial dissolved organic matter from peatlands on Sumatra. In contrast, coral cores from two other sites located far from major terrestrial organic matter sources show lower green-to-blue values and no convincing correlation with fluorescence intensity of terrestrial humic substances in the skeleton. Abiogenic aragonite precipitation experiments with both terrestrial and marine organic matter sources confirmed that terrestrial humic substances are readily incorporated into aragonite, but not fluorescent organic matter from marine sources. The results of this study suggest that in coral cores with high luminescence green-to-blue ratios (> 0.6) and large downcore variability (range of ≥ 0.05), the green-to-blue ratio is strongly linked to variation in terrestrial humic substances. Coral cores therefore have the potential to reconstruct past variation in terrigenous dissolved organic carbon fluxes.

Downstream persistence of particulate organic carbon released from thaw slumps on the Peel Plateau, NT, Canada

&lt;p&gt;Underlain by ice-rich permafrost, the Peel Plateau in western Canada is highly susceptible to rapid permafrost degradation in the form of retrogressive thaw slumps and has experienced a non-linear intensification in the area, volume, and thickness of permafrost thawed since 2002. These slumps tend to occur along stream networks, which flow directly into the Peel River, through the Mackenzie Delta, and into the Beaufort Sea. Thus, lateral transport of previously sequestered organic carbon from these features has the potential to propagate far downstream. Upstream-downstream comparisons have shown that thaw slumps mobilize material to stream systems primarily in the form of particulate organic carbon (POC), increasing organic carbon yields by orders of magnitude, and switching stream networks to particle-dominated systems. Furthermore, the bulk POC released from slumps can be upwards of 10,000 &lt;sup&gt;14&lt;/sup&gt;C years old, and base-extracted fluorescence measurements suggest material is more reworked since terrestrial production compared to upstream material.&lt;/p&gt;&lt;p&gt;To determine how far this effect propagates downstream we measured particulate and dissolved organic carbon (DOC) fluxes across stream transects extending 0.4 to 1 km downstream of thaw slumps in 1&lt;sup&gt;st&lt;/sup&gt; to 2&lt;sup&gt;nd&lt;/sup&gt; order streams and found no consistent decrease in TSS or POC fluxes with transit downstream. &amp;#160;In addition, we measured the composition (%POC, C:N, fluorescence, D&lt;sup&gt;14&lt;/sup&gt;C) and flux of DOC and POC within the ~1100 km&lt;sup&gt;2&lt;/sup&gt; Stony Creek watershed, examining tributary streams representing different vegetative, slump-density, and geological units in addition to the Stony Creek mainstem, to determine contributions to downstream flux. We found organic carbon fluxes were dominated by slump-mobilized POC at all points downstream of disturbance, and that these organic carbon fluxes were greater than any non-disturbed tributary stream. The &lt;sup&gt;14&lt;/sup&gt;C age of POC along the Stony Creek mainstem increased by thousands of years with the introduction of slump inputs and remained similarly depleted in &lt;sup&gt;14&lt;/sup&gt;C at the watershed outlet. Using historical suspended sediment, POC, and discharge data for the 75,000 km&lt;sup&gt;2&lt;/sup&gt; Peel River drainage basin containing the Stony Creek watershed, we will examine whether there have been increases in instantaneous sediment and POC fluxes during the thaw season to track the trends of intensifying slump activity that have been documented on the Peel Plateau. Constraining the downstream effect of these abrupt, localized disturbances may improve detection and prediction of change that will likely cascade through the region over the coming decades.&lt;/p&gt;