Keith Lawrence

Annual fluxes of carbon from deforestation and regrowth in the Brazilian Amazon

The distribution of sources and sinks of carbon among the worlds ecosystems is uncertain. Some analyses show northern mid-latitude lands to be a large sink, whereas the tropics are a net source; other analyses show the tropics to be nearly neutral, whereas northern mid-latitudes are a small sink. Here we show that the annual flux of carbon from deforestation and abandonment of agricultural lands in the Brazilian Amazon was a source of about 0.2 Pg C yr-1 over the period 1989–1998 (1 Pg is 1015 g). This estimate is based on annual rates of deforestation and spatially detailed estimates of deforestation, regrowing forests and biomass. Logging may add another 5–10% to this estimate, and fires may double the magnitude of the source in years following a drought. The annual source of carbon from land-use change and fire approximately offsets the sink calculated for natural ecosystems in the region. Thus this large area of tropical forest is nearly balanced with respect to carbon, but has an interannual variability of ± 0.2 PgC yr-1.

Late Quaternary sea-surface temperatures in the western Coral Sea: Implications for the growth of the Australian Great Barrier Reef

Recent chronostratigraphic evidence suggests that the central Australian Great Barrier Reef formed within the past 780 k.y. Periplatform sediments of the same age recovered from the western Coral Sea record a progressive decrease in the δ18O of planktonic foraminifera to the present. Several investigators have proposed that this trend represents an appreciable late Pleistocene warming (∼4 °C) of ocean surface temperatures, which they posit catalyzed the growth of the Great Barrier Reef. Contrary to this hypothesis, we demonstrate using alkenone paleothermometry (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(U_{37}^{k{^\prime}}\) \end{document}) on sediments from Ocean Drilling Program (ODP) Site 820 that sea-surface temperatures (SSTs) in the western Coral Sea changed by ∼1.5 °C or less during the past ∼800 k.y. If the central Great Barrier Reef rose in the late Quaternary, it was therefore not due to a warming of SSTs. We explore whether a major moisture balance change and/or diagenetic alteration of calcareous microfossils can explain the higher δ18O values observed at depth in the planktonic δ18O record at ODP Site 820. Our results suggest that diagenesis provides a large isotopic overprint.