Selective Preservation and the Interpretation of Pollen Data from Hypoxic Marine Conditions: Evidence from Cariaco Basin Core MD03-2620

Oxic/anoxic transitions in marine cores raise questions about the interpretation of pollen data because pollen tend to be grouped with other calcareous organisms which disappear with the onset of oxic conditions. However, pollen grains are extremely resistant in diverse mediums, and their sensitivity to oxic environments has been asserted, but not proven. We present a robust calculation of pollen grains from hypoxic conditions and an oxic/anoxic transition zone in core MD02-2620 from the Cariaco Basin and correlate the pollen data with organic content (grey scale), Ti and Ca concentration, and sedimentation rate. We also consider the effect of upwelling and the contribution of riverine pollen. The oxic/anoxic transition is determined from lithological data. Our statistical analysis shows that the minimum of pollen concentration does not coincide with the maximum of Ti input and the minimum of Ca concentration in the sediment, but follows it with a 300-400 year lag. The composition of the pollen assemblages indicates low diversity and an abrupt decrease of arboreal elements, mostly from the lowland vegetation group, but also an increase in non-arboreal pollen and spores. These observations are inconsistent with the assumption that the pollen spectrum is controlled primarily by selective preservation due to onset of oxic conditions. In fact, the only manifestation of selective preservation is the disappearance of the smallest grains in any group, regardless of ecology. Comparison of our results with pollen data from other marine cores allows us to conclude that unusual pollen assemblages provide information about climate change despite some selective preservation.