Holocene Climate and Carbon Sequestration via Black Carbon Burial in Sediments

Abstract

Fire, a global process that depends on climate, volcanic activity, vegetation and human practices is not constant through time and varies at decadal, centennial and millennial time-scales. Wildfires that result in burning of large areas of land, may increase in frequency in the future as a result of impending global warming. Studies have shown a correlation between historically warmer climates and occurrences of wildfires. The increased occurrence of these fires may add carbon dioxide [CO[subscript]2(g)] to the atmosphere. However, such wildfires may also have led to CO[subscript]2(g) sequestration by formation of pyrogenic black carbon (BC) followed by its subsequent burial. In this manner, climate-driven wildfire occurrence and corresponding BC formation and burial is a negative feedback loop in the carbon cycle. Carbon sequestration responding to climate fluctuations through fire occurrences may have implications for climate and carbon cycle models, as well as future climate predictions. It is hypothesized that an increased abundance in BC formation (i.e., carbon sequestration) will be correlated with a drier environment; a drier climatic interval in the sedimentary record will correlate with a greater frequency of wildfires and pyrogenic BC abundance. This hypothesis was tested in two areas: coastal North Carolina and Lake Bosumtwi, Ghana. Geochronological analysis conducted on a sediment core from coastal NC suggests that the core encompasses a time interval of -1081 Common Era (CE) to 1979 CE. The abundance of BC in the core ranged from 4 % to 97 % and was related to established proxies of paleoclimate (temperature and drought). Furthermore, stable isotopes of BC, organic carbon (OC) and concentrations of polycyclic aromatic hydrocarbons (PAHs) were also used in an attempt to understand the type of biomass burned. The type of biomass burned ranges from C[subscript]4 plants (-1081 until 860 CE), to C[subscript]3 plants (860 CE to present). Also in the core from coastal NC, cycles of BC excursions relate to climate proxies on decadal and centennial scales. In a sediment core from Lake Bosumtwi, Ghana the BC abundance relative to TOC is elevated (~90 % of the TOC is BC) throughout the core. Stable isotopic signatures of BC and OC were also analyzed in the sediment core from Lake Bosumtwi. These analyses suggested that a shift in vegetation/change in environments occurred in western Africa; a shift from a drier climate to a moist climate occurred at about 6 cm depth in the core. There was not enough sediment to constrain geochronology in this core. Both study sites are sensitive to the Atlantic Ocean and other ocean-atmospheric teleconnections. For example, changes in the North Atlantic thermohaline circulation and the Intertropical Convergence Zone (ITCZ) could have a large influence on droughts and fires in coastal NC and western Africa. If a relationship between carbon sequestration and paleoclimate can be identified, a better understanding of how climate has affected global carbon cycle over geologic time can be established.