Divergent drivers of carbon dioxide and methane dynamics in an agricultural coastal floodplain: post-flood hydrological and biological drivers

Webb, Jackie R., Santos, Isaac R., Tait, Douglas R., Sippo, James Z., Macdonald, Ben C.T., Robson, Barbara, Maher, Damien T., and UNSPECIFIED (2016) Divergent drivers of carbon dioxide and methane dynamics in an agricultural coastal floodplain: post-flood hydrological and biological drivers. Chemical Geology, 440. pp. 313-325.

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Many coastal floodplains have been artificially drained for agriculture, altering hydrological connectivity and the delivery of groundwater-derived solutes including carbon dioxide (CO2) and methane (CH4) to surface waters. Here, we investigated the drivers of CO2 and CH4 within the artificial drains.of a coastal floodplain under sugarcane plantation and quantify the contribution of groundwater discharge to CO2 and CH4 dynamics over a flood event (290 mm of rainfall). High temporal resolution, in situ observations of dissolved CO2 and CH4, carbon stable isotopes of CH4 (delta C-13-CH4), and the natural groundwater tracer radon (Rn-222) allowed us to quantify. CO2, CH4 and groundwater dynamics during the rapid recession of a flood over a five day period. Extreme super-saturation of free CO2 ([CO2*]) up to 2,951 mu M (25,480% of atmospheric equilibrium) was driven by large groundwater input into the drains (maximum 87 cm day-(1)), caused by a steep hydraulic head in the adjacent water table. Groundwater input sustained between 95 and 124% of the surface [CO2*] flux during the flood recession by delivering high carbonate alkalinity groundwater (DIC = 10,533 mu M, similar to pH = 7.05) to acidic surface water (pH <4), consequently transforming all groundwater-derived DIC to [CO2*]. In contrast, groundwater was not a major direct driver of CH4 contributing only 14% of total CH4 fluxes. A progressive increase in CH4 concentrations of up to similar to 2400 nM day-(1) occurred as a combination of increased substrate availability delivered by post-flood drainage water and longer residence times, which allowed for a biogenic CH4 signal to develop. The progressive enrichment in delta C-13-CH4 values (- 70%. to-48%.) and increase in CH4 concentrations (46-2460 nM) support coupled production-oxidation, with concentrations and delta C-13 values remaining higher (2,798 nM and-47%.) than pre-flood conditions (534 nM and-55 parts per thousand) three weeks after the flood. Our findings demonstrate how separate processes can drive the aquatic CO2 and CH4 response to a flood event in a drained coastal floodplain, and the key role groundwater had in post-flood [CO2*] evasion to the atmosphere, but not CH4. (C) 2016 Elsevier B.V. All rights reserved.

Item ID: 58052
Item Type: Article (Research - C1)
ISSN: 0009-2541
Keywords: Wetland, Acid sulfate soils, Seepage, Greenhouse gas, Stable isotopes
Copyright Information: © 2016 Elsevier B.V. All rights reserved. Accepted Version: © 2016 Elsevier B.V. All rights reserved. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
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Author's post-print permitted on open access repository after an embargo period of 12 months.

Funders: Australian Research Council (ARC), Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Projects and Grants: ARC LE120100156, ARC DE140101733, ARC DE150100581, CSIRO OCE Postgraduate scholarship
Date Deposited: 17 Apr 2019 09:22
FoR Codes: 37 EARTH SCIENCES > 3707 Hydrology > 370704 Surface water hydrology @ 20%
37 EARTH SCIENCES > 3703 Geochemistry > 370304 Organic geochemistry @ 80%
SEO Codes: 96 ENVIRONMENT > 9602 Atmosphere and Weather > 960201 Atmospheric Composition (incl. Greenhouse Gas Inventory) @ 100%
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