Organic and black carbon 13C and 14C through the Santa Monica Basin sediment oxic‐anoxic transition

C A Masiello,E R M Druffel

doi:10.1029/2002gl015050

Abstract

Black carbon (BC) is a significant percentage of sedimentary organic carbon (SOC) at abyssal ocean sites, but its presence in shelf sediments is not well studied. Approximately 1600AD, Santa Monica Basin bottom waters shifted from oxic to very low oxygen (dysoxic) deposition conditions. Under oxic deposition conditions BC was 11 ± 4% of SOC, whereas after the overlying water became dysoxic (and sediments became anoxic), BC was 5.2 ± 1.2% of SOC. This shift may reflect the preferential remineralization of non‐black SOC under oxic conditions. There is an offset between BC and SOC 14C ages which changes with oxidation conditions, suggesting that BC storage is related to oxygen exposure and confirming a previously published report of the vulnerability of BC to sedimentary oxidation [Middelburg, 1999]. Terrestrial carbon is 17 ± 5% of total SOC in this core's anoxic region, and 31 ± 11% of this terrestrial carbon is BC.

Highlights

Title Organic and black carbon 13 C and 14 C through the Santa Monica Basin sediment oxic-anoxic transition
[3] We focus on Black carbon (BC) in coastal ocean sediments to understand the role of BC as a component of stored sedimentary organic carbon (SOC)
BC is on the order of 10% of the carbon buried in the deep ocean [Herring, 1985; Masiello and Druffel, 1998; Smith et al, 1973; Suman et al, 1997], suggesting feedbacks between biomass burning and ocean SOC storage, including the possibility that biomass burning is a long term carbon cycle sink as well as a short term carbon cycle source

Summary

Introduction

[2] ‘BC’ is the suite of refractory carbonaceous compounds remaining after biomass burning and fossil fuel combustion and is best understood as a geochemical continuum, ranging from partially charred biomass to submicron soot spheres. The BC production rate is currently greater than the removal rate of carbon to deep-sediments [Hedges and Keil, 1995]. Imbalances of this magnitude would eventually ($10,000 years) lead to geologically implausible perturbations in atmospheric O2. The geochemistry of one of the major terrestrial sources of carbon to this coastal region (Santa Clara River) is available [Masiello and Druffel, 2001]; making possible a more complete estimate of basinwide SOC and BC sources and sinks. Accelerator mass spectrometry (AMS), routinely measures Á14C to ±5%, making BC source identification possible

Study Site and Methods

Results and Discussion

Controls on the BC-SOCnBC Age Offset

Conclusions