RECORD OF THE SOUTHWEST MONSOON FROM GULF OF MEXICO SEDIMENT CORES

Summer monsoonal rains are an important source of moisture to portions of southwestern United States and northern Mexico. Changes in the amount and seasonal distribution of precipitation in this semi-arid region influence overall water supply and severity of wildfires. Historical records demonstrate that precipitation in the southwestern US is quite variable but the long-term (millennial-scale) variability of the monsoon system is not well known. Studies based on tree-rings provide highly resolved records of past conditions but determining seasonality of precipitation is challenging. In addition, these records usually represent short intervals of the Holocene and often reflect local conditions. Studies of pollen in lake sediments, lake shoreline deposits and vegetation in packrat middens are also available, but these records are often discontinuous, many are difficult to date, and they are sometimes contradictory.

Variations in the relative abundance of the planktic foraminifer Globigerinoides sacculifer in marine sediments from the Pigmy Basin in the Gulf of Mexico closely match a tree-ring record from El Malpais National Monument on the southern periphery of the Colorado Plateau in west-central New Mexico. The tree-ring record from El Malpais is sensitive to variations in the southwestern monsoon because precipitation in New Mexico occurs predominantly in the mid-summer to fall (June to October). The Pigmy Basin record confirms a severe multi-century drought centered at ~ 1600 calendar years BP as well as several multidecadal droughts that are present in the El Malpais record. The similarity of the Pigmy Basin and El Malpais records suggests that variations in G. sacculifer abundance in marine sediments from the Gulf of Mexico can be used as a proxy for constructing a highly resolved, well-dated, and continuous history of the southwest monsoon for the entire Holocene.