Abstract:
Mountainous islands of the Pacific Rim (such as New Zealand) purportedly deliver
up to 40% of the suspended sediment load and up to 35% of the riverine particulate
organic carbon (POC) load to the world's oceans. On the east coast of New
Zealand's North Island, the Waipaoa River drains a steep, 2205 km2 catchment
located on the active collisional East Coast Continental Margin. It has an annual
suspended sediment load of 15 Tg (15 x 1012 g), making up ~7% of New Zealand's
total yield to the Pacific Ocean, and a mean annual POC discharge to the Pacific
Ocean of 86.7 Gg (86.7 x 109 g). The annual loss of OC to the floodplain is ~9% of
this annual POC discharge (~7.8 Gg).
A range of analyses (including organic carbon content (%OC), stable carbon
isotopes (Delta 13C), radiocarbon (14C), carbon to nitrogen ratios (C/N)a and carbon
loadings (OC:SA)) were performed on correlative sediments from a transect of 7
cores from depositional sites located on the Waipaoa River floodplain and adjacent
continental shelf and slope. Results were used to determine biogeochemical
characteristics of organic carbon (OC) at a range of depositional sites during its
transfer from terrestrial source to marine sink, and how large floods impact OC
transfer to the marine environment.
The high temporal variability in OC content (0.2 to 3.5%) and different source
signatures (Delta 13C of -26.7 to -20.6% degrees) of Waipaoa River floodplain deposits
prevented the establishment of a clear benchmark signature for flood deposits that
may be recognisable in the marine sedimentary record. The high spatial and
temporal variability of floodplain sediment OC, combined with the areal extent of
floodplains within the catchment, indicates the appreciable modulating effect the
floodplain has on OC transfers to the ocean. Since extensive stopbanks were
constructed on the main floodplain since the 1940' s, sequestration of OC in
floodplain sediments has reduced by about half, increasing the overall efficiency of
the Waipaoa River in transferring terrestrial OC directly to the marine
environment.
Flood layers are preserved in the marine sedimentary record. Continental shelf
sediments indicate that during Cyclone Bola (March 1988, a rainfall event with a
>100 year return period), the extreme river discharge produced a hyperpycnal
(negatively buoyant) plume, preserved as a ~10 cm thick layer on the inner shelf and
a ~1 cm thick layer on the mid-shelf. The flood layer contains a significant amount
of terrestrially-sourced OC (up to 86% of total OC in >25 Mu m fraction) which
subsequently was rapidly buried by normal marine deposits (in which ~60% of OC
in >25 Mu m fraction is terrestrial), thereby preserving its strong terrestrial source
signature.
As sediments are physically and biologically processed at various depositional sites
across the continental shelf and slope, they lose some of their modern terrestrial
OC, and the concurrent addition of marine sourced OC results in the sediments
gaining a stronger marine biogeochemical signature (Delta 13C values increasing from -26.2% degrees for floodplain sediments to -21.6% degrees for upper continental slope sediments).
Carbon loading (OC:SA) and 14C data revealed the contributions of kerogen,
modern terrestrial OC and modern marine OC to the total OC of continental shelf
and slope surface sediments. Sediments retain about 40% of their terrestrial OC
following transport to the continental slope, of which a significant amount consists
of kerogen. Because of high erosion rates within the catchment, kerogen associated
with the particles escapes oxidation, and therefore makes up a large part of the POC
flux. Kerogen is preserved across the margin to the mid-slope, where only 8% of the
bulk sediment OC consists of modern terrestrial OC, 58% is modern marine OC
and 34% is kerogen. Biomarker analyses of surface samples also support findings
that terrestrial OC is being transferred across the continental margin, with plant
sterols, long chain alcohols and long chain fatty acids (biomarkers indicative of
vascular plants) persisting as far offshore as the mid-continental slope.
Results presented verify and add to the understanding of OC transfers and
transformations at a range of depositional sites from terrestrial source to marine
sink. This study provides the first quantitative assessment of land to ocean OC
transfers from New Zealand. These findings, together with information on sediment budgets and depositional rates of OC in terrestrial and marine depositional
environments, could provide a vital step toward establishing global OC budgets for
small mountainous island environments.