Geology topics

Zhaohui Aleck Wang

Publications and source records attributed to Zhaohui Aleck Wang.

7 recordsLinked to original sources

USGS2023

Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes

Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol m −2 d −1 in mangroves and 57 ± 104 mmol m −2 d −1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism.

Nature↗

USGS2023

Practical guide to measuring wetland carbon pools and fluxes

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions.

Wetlands↗

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↗

USGS2018

Tidal Wetlands and Estuaries

1. The top 1 m of tidal wetland soils and estuarine sediments of North America contains 1,886 ± 1046 teragrams of carbon (Tg C). [High confidence, Very likely] 2. Soil carbon accumulation rate (i.e., sediment burial) in North American tidal wetlands is currently 9 ± 5 Tg C per year and estuarine carbon burial is 5 ± 3 Tg C per year. [High confidence, Likely] 3. The lateral flux of carbon from tidal wetlands to estuaries is 16 ± 10 Tg C per year for North America. [Low confidence, Likely] 4. In North America, tidal wetlands remove 27 ± 13 Tg C per year from the atmosphere, estuaries outgas 10 ± 10 Tg C per year to the atmosphere, and the net uptake by the combined wetland-estuary system is 17 ± 16 Tg C per year. [Low confidence, Likely] 5. Research and modeling needs are greatest for understanding responses to accelerated sea level rise, mapping tidal wetland and estuarine extent and quantification of CO2 and CH4 exchange with the atmosphere, especially in large, under-sampled, and rapidly changing regions. [High confidence, Likely] Note: Confidence levels are provided as appropriate for quantitative, but not qualitative, Key Findings and statements.

Book chapter↗

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↗