Postglacial Response of Terrestrial Neotropical Vegetation to Abrupt Climate Change as Recorded by Pollen from a Marine Core, Cariaco Basin

The response of terrestrial vegetation to the climatic shift that followed the Last Glacial Maximum is a critical component of the Neotropical climate system: it is linked to the carbon cycle and makes it possible to trace the impact of climatic alterations. We analyzed fossil pollen from high-resolution marine core MD03-2620 from the anoxic Cariaco Basin, off the coast of Venezuela. The study covers the period from Last Glacial Maximum (LGM) to the mid-Holocene. Previous paleoreconstructions from Cariaco Basin sediments emphasized that the abrupt climatic change that followed the LGM was associated with a shift of the ITCZ, subsequent sea level oscillations and alteration in a thermohaline circulation. We maintain that pollen from the marine core reflects vegetation trends of a large regional area, which smooths out local peculiarities in vegetation and allows us to trace the Cold/Dry-Warm/Wet dynamic in the pollen assemblages of the Cariaco sediments. Our pollen analysis indicates an interval of clear dominance of C4-type plants between ca 19.0 - 17.5 kyr BP, representing the transition from salt-marshes to steppe/savanna on the lowland. The onset of the Mystery Interval MI (17.5 to ~14.5 kyr BP) caused the most dramatic changes in vegetation for all postglacial time and was characterized sequentially by both a wet and dry signal, resulting in the reduction of forest vegetation, and later with the expansion of salt marshes in the littoral zone. According to the pollen data, MI consisted of two clearly recognized parts with a bridge in the middle: 1) H1-a. Dry interval between ~17.5 and 16.5 kyr BP with gradually growing humidity, that reaches a plateau at ~16.6 kyr BP. 2) Middle of the MI: ~ 16.5 - 15.8 kyr BP, exhibiting warm, humid conditions. 3) H1-b. Dry period after ~15.8 kyr BP, to ~14.5 kyr BP. The largest abrupt change in the pollen record is the transition to the Bølling/Allerød Stade, where our results are well correlated with plant wax biomarker analyses (Hughen at al, 2004) that show a clear trend for a reduction of C4 terrestrial vegetation and the growing dominance of C3 lowland vegetation and diverse forests. The switch back to C4 plants occurs in the pollen data at the transition to the Younger Dryas at ~ 13.0 kyr BP. At ca 11.5 kyr BP the trend reverses back to C3 dominant vegetation and growing humidity. The strong correlation of the pollen assemblages and pollen concentration with terrigenous input, calcium carbonate concentration and sedimentation rate, supports the validity of the pollen data and demonstrates that more detailed pollen study of these marine sediments will allow us to reconstruct a fuller picture of the contribution of plants to climatic cycles and especially their role in the carbon system during periods of abrupt climate change.