D. E. Black

Planktonic foraminiferal response to the 1997–1998 El Niño: A sediment-trap record from the Santa Barbara Basin

Here we report results on a sediment-trap study from the Santa Barbara Basin in which we examine the response of planktonic foraminifera to El Nino conditions. Planktonic foraminifera census counts were performed on biweekly sediment-trap samples collected over the interval spanning late summer 1995 through early spring 1998, a period that captured the transition from “normal” conditions in 1996 to full El Nino conditions during early 1997 through early 1998. Intra-annual variations in the foraminifera assemblage reflect changes in seasonal climate variability, significant differences occurring in both foraminiferal relative abundance and flux values between non–El Nino and El Nino years. Warmer waters in the basin during late 1997 were reflected by a significantly increased presence of warmer-water species generally absent under normal conditions. Surprisingly, despite greatly reduced upwelling conditions, foraminiferal flux values during the spring 1997 upwelling season were more than four times greater than the year before. The contrast between normal and El Nino foraminifer populations has important implications for reconstructing the past history of both El Nino events and more general climate histories from the Santa Barbara Basin varved sediment record.

Persistent drying in the tropics linked to natural forcing

Approximately half of the worlds population lives in the tropics, and future changes in the hydrological cycle will impact not just the freshwater supplies but also energy production in areas dependent upon hydroelectric power. It is vital that we understand the mechanisms/processes that affect tropical precipitation and the eventual surface hydrological response to better assess projected future regional precipitation trends and variability. Paleo-climate proxies are well suited for this purpose as they provide long time series that pre-date and complement the present, often short instrumental observations. Here we present paleo-precipitation data from a speleothem located in Mesoamerica that reveal large multi-decadal declines in regional precipitation, whose onset coincides with clusters of large volcanic eruptions during the nineteenth and twentieth centuries. This reconstruction provides new independent evidence of long-lasting volcanic effects on climate and elucidates key aspects of the causal chain of physical processes determining the tropical climate response to global radiative forcing.

Human adaptation strategies to abrupt climate change in Puerto Rico ca. 3.5 ka

The connection between climatic change and social response is complex because change articulates a number of inter-related factors. Human decisions are filtered by social buffers – including social memory, risk perception, and cultural priorities – and the rate and scale of climate change is usually much larger than the scale of human decision-making. In this article, we provide information on climate change based on precisely dated speleothems with the response evident in archaeological sites that have radiocarbon date ranges within the same time frame. A stalagmite recovered from within the catchment area for aquifer recharge of the Pre-Arawak site of Angostura in Barceloneta, Puerto Rico, shows that a significant wet period occurred between 3.9 and 3.1 ka (primarily centered at 3.5 ka). We investigate the effect that this increase in precipitation had on the earliest occupations on the island in the context of palaeoenvironmental, geoarchaeological, and archaeological records from Angostura, Maruca, and Paso del Indio. Our analysis suggests the presence of two different adaptation strategies: settlement relocation and microlandscape modification. Our study concludes that the social response to change cannot be seen as monolithic given that human behavior, even within the same period, addresses the needs of individual groups with different priorities. This multiplicity of responses can indeed enhance resilience as social support can continue through alliances and exchanges, strengthening social bonds that can help buffer catastrophes. The results can help shed light on the range of adaptation strategies to change encompassed within the manifestations of social resilience or vulnerability.