GeoPRISMS (2012 – 2016)
The Subduction Margin Carbon Cycle: A Preliminary Assessment of the Distribution Patterns of Multicycle Carbon(NSF, OCE-1144483)
The accretionary wedge portion of a subducting margin creates an intersection of multiple C-cycles that operate on vastly different timescales. This is largely unexplored aspect of the global C-cycle. Ancient (fossil rock) C is liberated via uplift and mass wasting, and mixed with more contemporary forms of organic C in surface environments. As the multicycle C mixtures are dispersed throughout the sedimentary system their components will express different behaviors reflecting their respective compositions and reactivities. The presence of the fossil C component has not been considered in most studies of active margin. We propose exploratory studies of the three GeoPRISMS Subduction Cycles and Deformation (SCD) primary focus sites, the Alaskan, Cascadia, and Hikurangi Margins, with the specific objective of developing a preliminary assessment of the distribution of multicycle C at the sites. Samples will come primarily from archives held by the National Institute of Water and Atmospheric Research of New Zealand and the Ocean Drilling Program repository in the US. In addition, sample collection and analysis is proposed for the planned Integrated Ocean Drilling Program Expedition 341. Analyses of samples will include Raman and FTIR microscopy, stable carbon and radiocarbon isotopic measurements, and elemental (H/C) determinations.
Differences through time in the volume and nature of exported organic carbon from terrestrial environments have implications for the regulation of the carbon cycle and can also provide information on storm frequency, sea level, precipitation regimes, vegetation type, erosion rate, climate shift, and tectonic movement/uplift. The Southern Alaska Margin represents an ideal active margin system in which to study the relationship between tectonic and glacial changes and ancient carbon export through time. As a preliminary study of these processes, I am using sample material from Site U1417 and rock samples of the Yakutat Terrane to investigate variation in sedimentary provenance to the distal Surveyor Fan. Initial interpretations of this material suggest at least three distinct sedimentary packages linked to the tectonic convergence of the Yakutat Terrane and the onset of glaciation. Observations of discrete coal and plant fragments, coupled with initial shipboard measurements, imply good preservation of a traceable terrestrial organic carbon signal at this site.
Organic carbon burial in active margin systems, especially those coupled with small
mountainous rivers, is particularly efficient (Blair and Aller, 2012). Furthermore, within the
organic fraction exist multiple pools of carbon, ranging in age from contemporary to millions years (Fig. 1) (Komada et al., 2005; Leithold et al., 2006; Trumbore, 2009; Blair and Aller, 2012). Cycles of uplift and erosion of accretionary wedge sediments lead to the formation of old pools, which merge during erosion and transport processes with younger material to form multicycle carbon. Of these sub-fractions, ancient carbon, also known as kerogen, comprises 25 – 45% of the organic carbon buried on active margin shelves and slopes (Blair et al., 2004). Investigations of the origin and ultimate fate of organic carbon in active margin systems have been conducted for decades though are mostly limited to the continental shelf and slope utilizing cores sampling tens of thousands of years (eg. – Prahl et al., 1994; Leithold et al., 2006; Brackley, 2006; Blair et al., 2010). The objective of this proposal is to approach unanswered questions about the role of ancient carbon in the active margin carbon cycle by extending studies further offshore along the trajectory of sediment transport and sample into deeper time.