Comments on the importance of black carbon in the global carbon cycle

GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L07504, doi:10.1029/2004GL019512, 2004 Oceanic export of relict carbon by small mountainous rivers Tomoko Komada, Ellen R. M. Druffel, and Susan E. Trumbore Department of Earth System Science, University of California Irvine, Irvine, California, USA Received 16 January 2004; revised 8 March 2004; accepted 17 March 2004; published 15 April 2004. [ 1 ] Small mountainous rivers (SMR) export globally significant quantities of particulate organic carbon (POC) to the oceans, the composition of which may be distinct from POC delivered by major world rivers. To better constrain the controls on the D 14 C signatures of POC suspended in SMR, we examined the temporal variability of D 14 C and d 13 C of POC suspended in the Santa Clara River, a SMR in California, USA. Our data suggest mixing of bedrock and contemporary soil, which can be applied to other SMR in contrasting climate zones and with varying land cover. The data also suggest that chemical denudation is incomplete, but effective to the point that only the most recalcitrant moieties of the parent POC are exported. We further propose sediment yield as a potentially useful proxy for the D 14 C of riverine POC. A preliminary analysis of the existing data for a spectrum I NDEX T ERMS : 1030 of rivers supports this hypothesis. Geochemistry: Geochemical cycles (0330); 1625 Global Change: Geomorphology and weathering (1824, 1886); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4806 Oceanography: Biological and Chemical: Carbon cycling; 4825 Oceanography: Biological and Chemical: Geochemistry; K EYWORDS : shale, weathering, radiocarbon, organic, suspended, sediment. Citation: Komada, T., E. R. M. Druffel, and S. E. Trumbore (2004), Oceanic export of relict carbon by small mountainous rivers, Geophys. Res. Lett., 31, L07504, doi:10.1029/ 2004GL019512. 1. Introduction [ 2 ] Small mountainous rivers (SMR; basin area < 10,00 0 km 2 , headwater height 1000 – 3000 m) have been projected to deliver as much as 40% of the global riverine sediment load due to their high erodibility [Milliman and Syvitski, 1992]. Correspondingly, 17– 35% of the global riverine particulate organic carbon (POC) load has been attributed to SMR [Lyons et al., 2002]. Compared to larger and/or lower-relief counterparts, POC suspended in SMR can be enriched in rock-derived organic carbon (OC), making SMR potentially significant sources of relict OC to the oceans [Kao and Liu, 1996; Masiello and Druffel, 2001; Blair et al., 2003]. The riverine flux of relict OC is a key component of the marine and global C cycles, influencing our understanding of the source of 14 C-depleted C in the oceans [Druffel et al., 1992], as well as controls on atmospheric oxygen over geologic time [Berner, 1989]. However, the controls on the isotopic composition of POC suspended in SMR are not well understood. [ 3 ] We report the temporal variability of D 14 C and d 13 C of POC suspended in the Santa Clara River, one of many SMR in southern California, USA. Previous work has Copyright 2004 by the American Geophysical Union. 0094-8276/04/2004GL019512 shown that D 14 C values of POC suspended in the Santa Clara can vary temporally, but on average, $40% of the POC may be derived of ancient C [Masiello and Druffel, 2001]. In the present study, we examined the isotopic composition of POC suspended in recent storm events, and compared them to potential end-member POC sources to identify the major controls on POC isotopic composition. We present two models—weight-% POC and sediment yield models—to better understand the variability of D 14 C signatures of POC suspended in rivers of different morphologies. 2. Study Sites and Methods [ 4 ] The Santa Clara is a mixed land-use SMR (drainage area 4210 km 2 ; headwater height 2900 m; Figure 1). Tectonic uplift, steep hillslopes, and friable bedrock pro- mote physical denudation and attenuate soil development. Semi-arid climate fosters surface vegetation dominated by mixed chaparral and sage scrub [Scott and Williams, 1978]. Precipitation is largely limited to the months of November through March, during which storms trigger discharge events. Such events are responsible for the annual discharge of both sediment and water [Warrick, 2002]. [ 5 ] During the winter of 2002 to 2003, suspended sedi- ment was collected under moderate to high flow conditions 0.3 km upstream of the river mouth using methods of Masiello and Druffel [2001]. The four events sampled accounted for 55% of the discharge for water year 2002 (Table 1). Samples were collected during low tide to avoid inclusion of marine POC, and stored on ice during transport to the laboratory. Within 24 hours of collection, suspended solids were concentrated from the river water by centrifu- gation. The solids were lyophilized, powdered, and stored frozen until analysis. Two samples collected in 1998 by Masiello and Druffel [2001] that had been stored frozen were also included in the current sample set (Table 1). Isotopic signatures reported by Masiello and Druffel [2001] are not used here, because of the possibility of incomplete carbonate removal (T. Komada, unpublished data, 2003). [ 6 ] Of the various sedimentary rocks that outcrop within the watershed, three were targeted as potential POC sources based on their outcrop areas and friability: Pico (Tp; Plio-Pleistocene), Monterey and lower Monterey (Tm and Tml; Miocene), and Cozy Dell (Tcd, Eocene). Tp has been proposed as a major source of sediment to the Santa Clara due to its exceptional erodibility [Warrick, 2002]. The remaining formations are less friable, but outcrop widely. We collected two sets of Tm samples: a dark, less weathered sample (Tm-black), and a lighter, more weathered sample (Tm-white). All samples were collected from 10 to 20 cm beneath road-cut surfaces avoiding surface litter and soil. All samples were powdered, lyophilized, and stored frozen until analysis. L07504 1 of 4