277-8 From Ashes to Atoms: Wildfire Impacts on Organic Carbon and Meteoric 10Be Systematics in the Critical Zone

Session: Critical Zone Science: Intersection of Processes Linked to Geomorphology, Ecology, Fire and Climate

Presenting Author:

Adrian Wackett

Authors:

Wackett, Adrian A.¹, Moreland, Kimber C.², Hirschey, Sarah H.³, Peña, Jasquelin⁴, Steelquist, Aaron T.⁵, Siirila-Woodburn, Erica R.⁶, Newcomer, Michelle E.⁷, Hidy, Alan J.⁸, Willenbring, Jane K.⁹

Abstract:

Wildfire burn area has quadrupled across the US since 1980. Nonetheless, it remains unclear how high-severity fire impacts soil organic carbon (SOC) and isotopic tracers like meteoric ¹⁰Be in the deep soils (>30 cm) and saprock that volumetrically dominate the critical zone. We hypothesized that wildfire modifies SOC and meteoric ¹⁰Be systematics in topsoils but not within the deeper (>30 cm) soil and saprolite layers shielded from direct heating and post-fire erosion. We collected 95 samples from a replicated (n=6) series of 15-m soil and saprolite boreholes into Miocene andesites and 122 stream water samples (Oct–Jan, daily; Feb–Sept, weekly) at the outlet of the Cosumnes River watershed prior to and one year after the 2021 Caldor Fire engulfed ~900 km² of Sierra Nevada forests. Our results reveal a tight coupling (R²=0.91) between SOC and meteoric ¹⁰Be throughout the entire 15+ meters of both pre- and post-fire weathering profiles, as well as significant shifts in the depth distribution and variance of SOC and meteoric ¹⁰Be concentrations post-fire. Key changes include ~40% enrichments in SOC and meteoric ¹⁰Be concentrations within the upper 0.5 m of soils and concomitant reductions from 3–7 m depth, leading to significantly (P<0.05) lower profile-integrated SOC and meteoric ¹⁰Be inventories in post-fire regolith. We show that fire-induced mobilization of SOC and meteoric ¹⁰Be from upland regolith generates a distinct signature in the dissolved load of Cosumnes River waters that persists for multiple years post-fire. Dissolved organic carbon (DOC) concentrations in post-fire peak flow greatly exceed historical levels, likely reflecting physical transport of SOC (and meteoric ¹⁰Be) enriched ash from burned hillslopes to channels via overland flow during precipitation events. Elevated DOC concentrations at base flow discharges may alternatively reflect the lateral percolation of SOC liberated from deep soils and saprock by fire-induced hydrologic/redox transformations in pore waters. Our findings highlight the need to constrain the (in-)direct biogeochemical impacts of wildfire on carbon and other element cycles in the deep critical zone and adjacent waterways. We suggest high-severity wildfires play an important but previously unrecognized role in modulating SOC and Be behavior and should therefore be accounted for when utilizing meteoric ¹⁰Be as a tracer of surface processes in wildfire-affected landscapes, which cover the vast majority of earth’s terrestrial surface.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9353

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.