| Climate is the primary factor affecting the distribution and productivity of life on Earth. Here I investigate the feedbacks between climate variability and biosphere response, by analyzing the isotopic composition of lake sediments and tree cores over a range of time scales.; I present a continuous 400,000-year record of precipitation variability from Lake Titicaca (16° S and 69° W) that reveals a dominant 100,000-year periodicity. During interglacial low-stands the accumulations of organic carbon increased by a factor of 10 to 20 and the organic nitrogen by a factor of 3 to 10. Paradoxically, an increase in the ratio of nitrogen isotopes (&dgr; 15N), conventionally interpreted as denitrification, also occurred during this period. I present an alternative mechanism, invoking nitrogen use efficiency in the water column, to explain this positive relationship between &dgr; 15N and total nitrogen in lake sediments.; In order to quantify biotic responses to climatic variability on annual scales I developed a multi-proxy approach employing ring widths and isotopes of carbon and oxygen in tropical trees. This approach was validated by intra-annual oxygen isotopic variability in an Amazon tree for which monthly growth data were available. This analysis revealed that growth and oxygen isotopes are significantly responsive to precipitation variability. When applied to other taxa and regions of tropical South America, this approach proved successful at capturing precipitation variability associated with the El Nino Southern Oscillation (ENSO).; Collectively these studies indicate that climatic forcing can have profound effects on the biosphere over a range of time scales. |
