It has been shown that, during glacial stages, continental margins tend to preserve a higher proportion of terrigenous organic matter (OM) than during interglacial conditions (Jasper and Gagosian, 1990, 1993; Lyle et al., 1992; Goņi, 1997; Visser et al., 2004). However, no work has provided a high-resolution study of OM over a specific glacial–interglacial transition, which is critical to sustain the work performed to understand microbial degradation dynamics in relation to changing oxidative conditions and source of reduced carbon during these periods. Thus, the central aspect of this study was the generation of a detailed and high-resolution geochemical analysis of OM components preserved in a core from the Peruvian shelf (Ocean Drilling Program [ODP] Leg 201, Hole 1229E) during the last glacial–interglacial climatic transition. In addition, we have also studied the signature of specific biomarkers (lignins) that can provide further information on dominant terrigenous vegetation and land-based conditions during this transition.
Several authors (Brown, 1987; van der Hammen and Asby, 1994; Piperno, 1997; van der Kaars et al., 2000; Hope, 2001) have suggested that during the Last Glacial Maximum (LGM), savanna vegetation expanded at the expense of forest ecosystems in the Amazon and Indonesian Basins as the result of enhanced aridity of tropical to equatorial areas. Piperno (1997) even proposed that full-grown forest refuges might have existed along rivers and streams contributing disproportionate amounts of nongrass materials to the rivers and the Amazonian Fan during the LGM despite an overall reduction in the nonriverine forest ecosystems. Indeed, the importance of glacial aridification of tropical ecosystems is still being debated, since very few indicators of grasslike vegetation (i.e., molecular markers of grassy tissues) have been recorded in submarine fans draining the Amazon Basin during the LGM (Goņi, 1997; Haberle, 1997; Kastner and Goņi, 2003). Using similar vascular biomarker signatures, Visser et al. (2004) demonstrated that in the Southeast Asian tropics, an increase in winter monsoons during the LGM generated moist conditions that allowed tropical rainforests to vegetate exposed shelves during glacial maxima, contributing a continuous source of tree-derived materials to the shelf sediments as opposed to grass materials, which would be expected from savanna-dominated systems under drier conditions.
Continental margins thus have the potential to record glacial–interglacial fluctuations in the marine vs. terrigenous sources of OM, as well as variations in the composition of the terrigenous OM pool. We present here elemental, isotopic, and biomarker data in a core from the Peruvian margin spanning the late Pleistocene through the Holocene.
Since the Peruvian margin is a site of major upwelling and receives very little riverine discharges, the composition of its sediments should be disproportionately controlled by marine sources. Terrigenous components should thus contribute only a minor proportion of the total OM pool and a much lower one than in similar high-productivity systems such as that of the western African margin (Farrimond et al., 1990b). Hence, this may be an ideal setting for the study of glacial–interglacial shifts in OM signatures of predominant marine origin and to test if this shelf system still receives similar trends in terrigenous OM influx, across the LGM to the present, as was reported for other major shelf environments.