Rosenheim Brad E.

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Rosenheim
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Brad E.
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Subject: Carbon cycle ×

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  • Article
    Direct measurement of riverine particulate organic carbon age structure
    (American Geophysical Union, 2012-10-03) Rosenheim, Brad E. ; Galy, Valier
    Carbon cycling studies focusing on transport and transformation of terrigenous carbon sources toward marine sedimentary sinks necessitate separation of particulate organic carbon (OC) derived from many different sources and integrated by river systems. Much progress has been made on isolating and characterizing young biologically-formed OC that is still chemically intact, however quantification and characterization of old, refractory rock-bound OC has remained troublesome. Quantification of both endmembers of riverine OC is important to constrain exchanges linking biologic and geologic carbon cycles and regulating atmospheric CO2 and O2. Here, we constrain petrogenic OC proportions in suspended sediment from the headwaters of the Ganges River in Nepal through direct measurement using ramped pyrolysis radiocarbon analysis. The unique results apportion the biospheric and petrogenic fractions of bulk particulate OC and characterize biospheric OC residence time. Compared to the same treatment of POC from the lower Mississippi-Atchafalaya River system, contrast in age spectra of the Ganges tributary samples illustrates the difference between small mountainous river systems and large integrative ones in terms of the global carbon cycle.
  • Article
    Constraints on the timing and extent of deglacial grounding line retreat in West Antarctica
    (American Geophysical Union, 2023-04-26) Venturelli, Ryan A. ; Boehman, Brenna ; Davis, Christina ; Hawkings, Jon R. ; Johnston, Sarah E. ; Gustafson, Chloe D. ; Michaud, Alexander B. ; Mosbeux, Cyrille ; Siegfried, Matthew R. ; Vick‐Majors, Trista J. ; Galy, Valier ; Spencer, Robert G. M. ; Warny, Sophie ; Christner, Brent C. ; Fricker, Helen A. ; Harwood, David M. ; Leventer, Amy ; Priscu, John C. ; Rosenheim, Brad E.
    Projections of Antarctica's contribution to future sea level rise are associated with significant uncertainty, in part because the observational record is too short to capture long‐term processes necessary to estimate ice mass changes over societally relevant timescales. Records of grounding line retreat from the geologic past offer an opportunity to extend our observations of these processes beyond the modern record and to gain a more comprehensive understanding of ice‐sheet change. Here, we present constraints on the timing and inland extent of deglacial grounding line retreat in the southern Ross Sea, Antarctica, obtained via direct sampling of a subglacial lake located 150 km inland from the modern grounding line and beneath >1 km of ice. Isotopic measurements of water and sediment from the lake enabled us to evaluate how the subglacial microbial community accessed radiocarbon‐bearing organic carbon for energy, as well as where it transferred carbon metabolically. Using radiocarbon as a natural tracer, we found that sedimentary organic carbon was microbially translocated to dissolved carbon pools in the subglacial hydrologic system during the 4.5‐year period of water accumulation prior to our sampling. This finding indicates that the grounding line along the Siple Coast of West Antarctica retreated more than 250 km inland during the mid‐Holocene (6.3 ± 1.0 ka), prior to re‐advancing to its modern position.