Century-scale records of land-based activities recorded in Mesoamerican coral cores.

The Mesoamerican Reef, the second-largest barrier reef in the world, is located in the western Caribbean Sea off the coasts of Mexico, Belize, Guatemala, and Honduras. Particularly in the south, the surrounding watersheds are steep and the climate is extremely wet. With development and agricultural expansion, the potential for negative impacts to the reef from land-based runoff becomes high. We constructed annually resolved century-scale records of metal/calcium ratios in coral skeletons collected from four sites experiencing a gradient of land-based runoff. Our proxy data indicate that runoff onto the reef has increased relatively steadily over time at all sites, consistent with land use trends from historical records. Sediment supply to the reef is greater in the south, and these more exposed reefs will probably benefit most immediately from management that targets runoff reduction. However, because runoff at all sites is steadily increasing, even distal sites will benefit from watershed management.

Abrupt tropical vegetation response to rapid climate changes.

Identifying leads and lags between high- and low-latitude abrupt climate shifts is needed to understand where and how such events were triggered. Vascular plant biomarkers preserved in Cariaco basin sediments reveal rapid vegetation changes in northern South America during the last deglaciation, 15,000 to 10,000 years ago. Comparing the biomarker records to climate proxies from the same sediment core provides a precise measure of the relative timing of changes in different regions. Abrupt deglacial climate shifts in tropical and high-latitude North Atlantic regions were synchronous, whereas changes in tropical vegetation consistently lagged climate shifts by several decades.

Climate and the collapse of Maya civilization.

In the anoxic Cariaco Basin of the southern Caribbean, the bulk titanium content of undisturbed sediment reflects variations in riverine input and the hydrological cycle over northern tropical South America. A seasonally resolved record of titanium shows that the collapse of Maya civilization in the Terminal Classic Period occurred during an extended regional dry period, punctuated by more intense multiyear droughts centered at approximately 810, 860, and 910 A.D. These new data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by abrupt drought events, contributing to the social stresses that led to the Maya demise.

Virus-specific adaptations for the production of a pseudorecombinant virus formed by two distinct bipartite geminiviruses from Central America.

Most whitefly-transmitted geminiviruses possess bipartite genomes comprising DNAs A and B. The production of viable pseudorecombinants by reassortment of infectious cloned components is generally limited to isolates/strains of a particular virus. Following exchange of cloned genomic components of Sida golden mosaic virus from Costa Rica (SiGMV/Co) and Sida golden mosaic virus from Honduras (SiGMV/Ho(yv)), the pseudorecombinant viruses were infectious in various plant species. Three DNA B components (B(1), B(2), B(3)), different in a few nucleotides, were isolated from Sida rhombifolia naturally infected with SiGMV/Ho(yv). Only SiGMV/Ho(yv) DNA B(2) was able to form a viable pseudorecombinant with SiGMV/Co DNA A. In protoplasts, as well as in inoculated leaves, SiGMV/Co DNA A trans-replicated the heterogenomic SiGMV/Ho(yv) DNA B(1) component, indicating that impaired movement is involved in the deficiency of SiGMV/Ho(yv) DNA B(1) to form a pseudorecombinant virus with SiGMV/Co DNA A. Even after extensive mutation analysis of SiGMV/Ho(yv) DNA B(1) and B(2), we were unable to pinpoint differences in SiGMV/Ho(yv) DNA B(2) that allowed the formation of a pseudorecombinant virus with SiGMV/Co DNA A. We observed a gradual increase of infectivity from noninfectious SiGMV/Co DNA A/SiGMV/Ho(yv) DNA B(1) and B(3) pseudorecombinant virus to pseudorecombinant viruses showing normal systemic spread of both genomic components associated with symptomatic plants.