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Terrestrial organic carbon storage in the Cariaco Basin catchment since the last glacial period

Summary

Introduction

The coastal ocean receives an estimated 0.5 Pg/y organic carbon (OC) from continental runoff (1). In addition to contributing to long-term regulation of atmospheric carbon dioxide and global climate, deposition and preservation of terrestrial OC in marine sediments can provide important archives of past environmental conditions on land over long time scales. I plan to use such archives in my thesis work to explore the relationship between climate change and the terrestrial carbon cycle. Specifically, I am investigating how climate change in the tropics, particularly hydrologic change, since the last glacial period affected the residence time of vascular plant-derived OC in “active” terrestrial reservoirs (i.e. soils, wetlands, river sediments) before deposition in marine or lacustrine sediments. Climate-induced changes in the storage time of biospheric OC in terrestrial reservoirs, for instance temperature control on microbial respiration in soil or the influence of precipitation on soil erosion and leaching, are part of a potentially important but poorly understood climate feedback loop. Characterizing the response of terrestrial systems to large climate changes in the past thus offers valuable perspective for assessing how the large amount of carbon stored on land will respond to present and future climate change.

While only a small fraction of terrestrial OC is preserved in sediments, on a first order this fraction is representative of the bulk pool of terrestrial OC. By tracking changes in the average age of terrestrial OC in sediments since the glacial period, I will be able to gain insight into the climatic controls on the speed of terrestrial OC cycling in different tropical drainage basins. With a well characterized and highly variable climate history, as well as an excellent sediment record, I have selected the Cariaco Basin catchment of northern Venezuela as one of two study sites to generate records of terrestrial residence time of vascular plant biomarkers over the last 20 ka. I am requesting $1278 for compound-specific 14C analysis to construct an initial record for the Cariaco Basin catchment at six key climate horizons. This part of my thesis is not supported by my advisors’ current funding, and this initial dataset will be extremely valuable for future proposals to secure funding for my thesis research.

Study Site

Cariaco Basin is an anoxic basin situated on the continental shelf off the central coast of Venezuela in the northern tropics (Figure 1). The basin receives continental runoff from several small mountainous and lowland rivers that empty directly onto the Unare Platform, a wide shallow (<100 m) shelf off the Venezuelan coast. The catchment consists of sparsely forested grassland in the Rio Unare drainage basin to rainforest and deciduous forest in the mountainous Rio Tuy basin. Precipitation is controlled by seasonal migration of the Intertropical Convergence Zone: the majority of annual precipitation (~900 mm/y) falls during the boreal summer/autumn when the ITCZ is furthest north, while the remainder of the year is dry (2).

Analysis of terrestrial biomarkers in sediments has yielded records of tropical climate during the last glacial cycle, including a series of rapid climate changes during the last deglaciation (3,4). These records show marked changes in precipitation over northern Venezuela due to long-term ITCZ migration in step with North Atlantic temperature variability over the past20 ka. As shown in Figure 1, the late glacial period and Younger Dryas (YD) stadial were marked by dry conditions, while precipitation increased during the Bølling-Allerød (BA) interstadial and Holocene interglacial. A trend towards drier conditions through the Holocene is coincident with decreasing summer insolation and southward ITCZ migration. Superimposed on these changes in precipitation is temperature increase of 3-6°C from the last glacial period to the Holocene (5).


Approach

In order to assess the effect of climate change on the average residence time of vascular plant-derived OC in the Cariaco catchment, I plan to use compound-specific 14C analysis to date terrestrial biomarkers at different horizons within sediment cores previously collected at Ocean Drilling Project Site 1002. The specific biomarkers I will use are long-chain (C26-C32) n-alkanoic acids, which are produced nearly exclusively by vascular plants as components of epicuticular leaf waxes (6) and serve as a proxy for terrestrial vegetative OC. Although biospheric OC is comprised of a wide variety of compounds that have been shown to cycle on different timescales (7), long-chain fatty acids are relatively resistant to biodegradation and are abundant in both fresh plants and soils, thus representing both the refractory and labile subset of biospheric OC. Critically, long-chain fatty acids are not preserved during rock formation nor produced during OC maturation, thus eliminating the risk of contamination from “fossil” petrogenic carbon. At the time of their production, vascular plant compounds have atmospheric 14C signatures that subsequently decay during storage in soil or other reservoirs. Thus, the offset between the 14C ages of leaf waxes and the sediment deposition age is a measure of the average residence time of these compounds in continental reservoirs. The sediment deposition age will be based on the previously developed core chronology based on varve counts in laminated sediments and foraminiferal radiocarbon ages (8).

For this initial survey I plan to focus on six key climate horizons as shown in Figure 1, including the late glacial period, BA, YD, early-, mid-and late Holocene. This sample set will be the foundation for a higher resolution record that I plan to generate for my thesis and provide the first insight into the influence of precipitation and temperature variability on the terrestrial storage of leaf wax compounds at this site.

References: [1] Bianchi, T.S. Proc Nat Acad Sci 108, 19473-19481 (2011). [2] University of East Anglia. 0.5 x 0.5degree 1961-1990 Monthly Climatology. [3] Hughen, K. A., Eglinton, T. I., Xu, L. & Makou, M. Science 304, 1955­1959 (2004). [4] Drenzek, N.J. MIT/WHOI PhD thesis (2007). [5] Farrera, I. et al. Clim Dyn 15, 823-856 (1999). [6] Eglinton, G. & Hamilton, R. J. Science 156, 1322 (1967). [7] Trumbore, S. E. Proc Nat Acad Sci 94, 8284-8291 (1997). [8] Hughen, K. A. et al. Science 303, 202 (2004).

Last updated: December 5, 2013