Geology topics

Sophie N. Chu

Publications and source records attributed to Sophie N. Chu.

4 recordsLinked to original sources

USGS2023

High-frequency variability of carbon dioxide fluxes in tidal water over a temperate salt marsh

Existing analyses of salt marsh carbon budgets rarely quantify carbon loss as CO 2 through the air–water interface in inundated marshes. This study estimates the variability of partial pressure of CO 2 ( p CO 2 ) and air–water CO 2 fluxes over summer and fall of 2014 and 2015 using high-frequency measurements of tidal water p CO 2 in a salt marsh of the U.S. northeast region. Monthly mean CO 2 effluxes varied in the range of 5.4–25.6 mmol m −2 marsh d −1 (monthly median: 4.8–24.7 mmol m −2 marsh d −1 ) during July to November from the tidal creek and tidally-inundated vegetated platform. The source of CO 2 effluxes was partitioned between the marsh and estuary using a mixing model. The monthly mean marsh-contributed CO 2 effluxes accounted for a dominant portion (69%) of total CO 2 effluxes in the inundated marsh, which was 3–23% (mean 13%) of the corresponding lateral flux rate of dissolved inorganic carbon (DIC) from marsh to estuary. Photosynthesis in tidal water substantially reduced the CO 2 evasion, accounting for 1–86% (mean 31%) of potential CO 2 evasion and 2–26% (mean 11%) of corresponding lateral transport DIC fluxes, indicating the important role of photosynthesis in controlling the air–water CO 2 evasion in the inundated salt marsh. This study demonstrates that CO 2 evasion from inundated salt marshes is a significant loss term for carbon that is fixed within marshes.

Massachusetts↗

USGS2020

An important biogeochemical link between organic and inorganic carbon cycling: Effects of organic alkalinity on carbonate chemistry in coastal waters influenced by intertidal salt marshes

Organic acid charge groups in dissolved organic carbon (DOC) contribute to total alkalinity (TA), i.e. organic alkalinity (OrgAlk). Its effect is often ignored or treated as a calculation uncertainty in many aquatic CO2 studies. This study evaluated the variability, sources, and characteristics of OrgAlk in estuarine waters exchanged tidally with a groundwater-influenced salt marsh in the northeast USA. Importantly, OrgAlk was found to serve as a biogeochemical medium linking organic and inorganic carbon cycling through its effects on pH, CO2 system speciation, and buffering capacity (H = -(∂pH/∂[H+])-1). Both the concentrations and characteristics of the identified organic acid charge groups, as well as water pH, influenced the magnitude and sign of the OrgAlk effects. The two main charge groups identified include carboxylic and phenolic or amine groups, with concentrations and pK values varying across tides and seasons. OrgAlk and DOC in the tidal creek were highly variable over tidal and seasonal cycles, and may be sourced from both terrestrial groundwater and in situ production in salt marsh sediments. OrgAlk seems to be more preserved over DOC in groundwater, although DOC and OrgAlk largely covaried in marsh tidal water, but with variable OrgAlk:DOC ratios. This highlights the insufficiency of using a fixed proportion of DOC to account for organic acid charge groups. OrgAlk was found to affect H+ concentrations by ~ 1 – 40 nmol kg-1 (equivalent to a pH change of ~ 0.03 – 0.26), pCO2 by ~ 30 – 1590 atm and buffering capacity by ~ 0.00 – 0.14 mmol kg-1 at relative OrgAlk contributions of 0.9 – 4.3% of TA observed in the marsh-influenced tidal water. Thus OrgAlk may have a significant influence on coastal inorganic carbon cycling. Further theoretical calculations confirm that these concentrations of OrgAlk would have sizable impacts on both carbonate speciation and, ultimately, air-sea CO2 fluxes in different coastal environments, ranging from estuarine to shelf waters. A new conceptual model linking organic and inorganic carbon cycling for coastal waters is proposed to highlight the sources and sinks of organic acid charge groups, as well as their biogeochemical behaviors and mechanistic control on the CO2 system.

Geochimica et Cosmochimica Acta↗

USGS2018

Deciphering the dynamics of inorganic carbon export from intertidal salt marshes using high-frequency measurements

The lateral export of carbon from coastal marshes via tidal exchange is a key component of the marsh carbon budget and coastal carbon cycles. However, the magnitude of this export has been difficult to accurately quantify due to complex tidal dynamics and seasonal cycling of carbon. In this study, we use in situ , high-frequency measurements of dissolved inorganic carbon (DIC) and water fluxes to estimate lateral DIC fluxes from a U.S. northeastern salt marsh. DIC was measured by a CHANnelized Optical Sensor (CHANOS) that provided an in situ concentration measurement at 15-min intervals, during periods in summer (July – August) and late fall (December). Seasonal changes in the marsh had strong effects on DIC concentrations, while tidally-driven water fluxes were the fundamental vehicle of marsh carbon export. Episodic events, such as groundwater discharge and mean sea water level changes, can impact DIC flux through altered DIC concentrations and water flow. Variability between individual tides within each season was comparable to mean variability between the two seasons. Estimated mean DIC fluxes based on a multiple linear regression (MLR) model of DIC concentrations and high-frequency water fluxes agreed reasonably well with those derived from CHANOS DIC measurements for both study periods, indicating that high-frequency, modeled DIC concentrations, coupled with continuous water flux measurements and a hydrodynamic model, provide a robust estimate of DIC flux. Additionally, an analysis of sampling strategies revealed that DIC fluxes calculated using conventional sampling frequencies (hourly to two-hourly) of a single tidal cycle are unlikely to capture a representative mean DIC flux compared to longer-term measurements across multiple tidal cycles with sampling frequency on the order of tens of minutes. This results from a disproportionately large amount of the net DIC flux occurring over a small number of tidal cycles, while most tides have a near-zero DIC export. Thus, high-frequency measurements (on the order of tens of minutes or better) over the time period of interest are necessary to accurately quantify tidal exports of carbon species from salt marshes.

Marine Chemistry↗

USGS2016

Intertidal salt marshes as an important source of inorganic carbon to the coastal ocean

Dynamic tidal export of dissolved inorganic carbon (DIC) to the coastal ocean from highly productive intertidal marshes and its effects on seawater carbonate chemistry are thoroughly evaluated. The study uses a comprehensive approach by combining tidal water sampling of CO 2 parameters across seasons, continuous in situ measurements of biogeochemically-relevant parameters and water fluxes, with high-resolution modeling in an intertidal salt marsh of the U.S. northeast region. Salt marshes can acidify and alkalize tidal water by injecting CO 2 (DIC) and total alkalinity (TA). DIC and TA generation may also be decoupled due to differential effects of marsh aerobic and anaerobic respiration on DIC and TA. As marsh DIC is added to tidal water, the buffering capacity first decreases to a minimum and then increases quickly. Large additions of marsh DIC can result in higher buffering capacity in ebbing tide than incoming tide. Alkalization of tidal water, which mostly occurs in the summer due to anaerobic respiration, can further modify buffering capacity. Marsh exports of DIC and alkalinity may have complex implications for the future, more acidified ocean. Marsh DIC export exhibits high variability over tidal and seasonal cycles, which is modulated by both marsh DIC generation and by water fluxes. The marsh DIC export of 414 g C m −2 yr −1 , based on high-resolution measurements and modeling, is more than twice the previous estimates. It is a major term in the marsh carbon budget and translates to one of the largest carbon fluxes along the U.S. East Coast.

Massachusetts↗