Australia: The Land Where Time Began

A biography of the Australian continent 

Carbon Dioxide Sequestering in Botany Bay

It is a well-known phenomenon that coastal marine environments offshore from human habitation suffer losses of marine benthic macrophytes, such as sea grasses, as well as increased microalgal production as a result of eutrophication. A not well-known factor was whether the composition of detritus in the sediment change, any changes potentially impacting the role of such marine systems in carbon dioxide sequestering. Cores were drilled at 2 sites in Botany Bay, Sydney, the marine ecosystem offshore from the location of the first settlement of Australia by Europeans, so the location that had the longest association with a European type settlement in Australia.

The authors reconstructed the sedimentary history indicated by the cores from the 2 sites. They were looking for changes in the dominant sources of detritus in the sediment in the estuary that could be human-induced. The time period covered by the cores was from the Middle Holocene to the present, a period of about 6,000 years, as indicated by ²¹⁰Pb profiles and radiocarbon dating. Over the last 30-50 years sedimentation rates have increased considerably making it too difficult to characterise depositional histories at the sites by a linear sedimentation rate. C:N ratios began to decline about the time of European settlement, the sediments from this time exhibiting an increased microalgal source signature. The authors suggest this could be the result of increased nutrient flows into the bay as a result of anthropogenic activity, as indicated by stable isotope analysis of ¹²C/¹³C ratio. The relative contribution to the detritus in the sediment of seagrass and C3 terrestrial plants such as mangroves and saltmarsh, declined around the time of rapid expansion of industries in about the 1950s, as indicated by carbon isotope ratios. This coincided with an increase in the proportion of detritus contributed by microalgal sources.

The authors concluded that within Botany Bay the  relative contribution of microalgae to the detritus has increased in line with increasing industrialisation and concomitant eutrophication. The carbon sink potential of Botany Bay has been substantially weakened by the relative changes in detritus contribution to the sediment, as the carbon burial efficiency of microalgae is about 0.1 % and that of seagrasses and C3 terrestrial plants is about 10 %. The authors suggest that such changes are probably occurring globally. If this is the case it could substantially decrease the role of such inshore marine ecosystems in helping to ameliorate climate change.

Sources & Further reading

1. Mcready, P.I. et al., 2011, Paleoreconstruction of estuarine sediments reveal human-induced weakening of coastal carbon sinks, Global Change Biology. (Accepted  unedited article published online for future issues.