25th International Meeting on Organic Geochemistry IMOG 2011

O-39
The fate of terrestrial organic matter in the Yangtze river- East
China sea system and its implications for the use of terrestrial
organic proxies
Chun Zhu 1 , Thomas Wagner 2 , Helen Talbot 2 , Johan Weijers 1,3 , Richard Pancost 1
1 Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of
Bristol, Bristol, United Kingdom, 2 School of Civil Engineering and Geosciences, Newcastle University,
Newcastle, United Kingdom, 3 Utrecht University, department of Earth Sciences – Geochemistry, Utrecht,
Netherlands (corresponding author:czhu@uni-bremen.de)
Burial of organic carbon (OC) in marine
sediments represents the 2 nd largest sink of
atmospheric CO2 over geological time scales.
Although continental shelves receive massive OC
inputs, their role in OC accumulation vs. degradation
is still under debate. The Yangtze River-dominated
East China Sea (YR-ECS; Fig. 1) is characterized by
massive terrestrial inputs (the YR is the world‘s 4 th
largest river in terms of sediment discharge) onto a
well-ventilated, shallow (500 km)
continental shelf, distinct from most narrow shelves in
the world. We examine the unique role of the ECS on
the fate of terrestrial organic matter (TOM) and its
implications for the use of terrestrial biomarkers.
Highly degraded soil OM and organic pollutants
dominate the YR sediments. Such OM is degraded
effectively in the estuary, particularly in the turbidity
maximum zone, where an enhanced bacterial activity,
including the bloom of methane oxidizing bacteria, is
also recorded by higher concentrations and structural
diversity of bacteriohopanepolyols (BHPs). Although
TOM has been extensively degraded in the watershed
and estuary, degradation of TOM clearly continues
along the coast and across the shelf, which is driven
by constant coastal currents and intermittent bottom
currents. Due to strong winnowing, few riverdischarged
sediments are accumulated on the open
shelf whereas relict sands deposited during the low
sea level stands occur widely on the seafloor. Those
sands contain a persistent amount of relict OC (ca.
0.1%), which appears to be δ 13 C-depleted (ca. -24‰)
but terrestrial biomarker-absent. Relict OC on the
sandy open ECS shelf accounts for ~170% of the
particulate OC annually discharged by the YR. Given
the globally wide occurrence of relict sands, a
persistent amount of relict OC may complicate the
modern TOM budget on continental shelves.
The behaviour of novel soil bacterial biomarkers
including BHPs and GDGTs (glycerol dialkyl glycerol
tetraethers) in the YR-ECS system was examined.
BHP distribution from the watershed to the shelf edge
indicates a fundamental difference in hopanoidproducing
communities between soils (source) and
sediments (sink). The Rsoil index, based on the ratio of
soil-marker BHPs vs. bacteriohopanetetraol, is
proposed as a new approach to track soil OM
distribution in marine sediments. This index displays a
strong correlation (R 2 =0.75) with the BIT index and
moderate correlations with land plant biomarkerderived
proxies in this setting. Branched GDGT-based
proxies for soil pH and air temperature do not reflect
the actual catchment environmental conditions due to
their extensive degradation in the estuary and in-situ
production on the well-ventilated ECS.
Finally, the fidelity of a range of proxies for TOM
input is examined. All proxies showed similar land-sea
trends; however, two factors impact their applicability
significantly: 1) the distinct reactivity, with a
decreasing degradability order: soil-marker BHPs > αamyrin
> β-amyrin > taraxerol > lignin > HMW nalkanols
> HMW n-alkanes, and similarly, the Rsoil
changing faster than the BIT which changes faster
than δ 13 Corg; 2) the strong regional variations in
marine biomarker distributions, which complicate the
normalization of terrestrial signals. We thus suggest
using a multi-proxy approach, considering the nature
of biomarkers and the depositional settings, when
reconstructing OC supply on continental shelves.
Fig.1. Sampling sites of surface sediment (0-5 cm) in
the YR-ECS system
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