Gulf of California regional climate change during the past 55,000 years
Separated from the cool waters of the mid-latitude North Pacific by the Baja California Peninsula and extending for over 1000 km, the Gulf of California (GoC) provides a critical pathway for monsoon moisture to enter the southwest US from the tropical eastern Pacific. A high-resolution study of a sediment core collected in the central GoC spans the past 55,000 years and reveals that Holocene (past 11,500 years) surface water conditions are unique. This implies that, before the Holocene, summer monsoonal moisture likely did not penetrate into the southwest US along the modern pathway through the GoC. Prior to the Holocene, tropical monsoon moisture appears to have taken a more direct pathway into the southwest US over Pacific waters off Baja California that were likely warmer than those of the GoC.
The study used fossilized remains of diatoms and silicoflagellates, two groups of marine microorganisms that are reliable indicators of past climate conditions. The results of this investigation indicate that temperatures and biologic productivity in central GoC waters varied greatly from about 55,000 years ago until the beginning of the Holocene interglacial and are related to global climate conditions.
For example, between about 55,000 and 27,000 years ago, numerous periods of massive iceberg creation (Heinrich events) were associated with rapid collapse of the northern hemisphere ice shelves. These events resulted in the release of large volumes of freshwater that in turn led to the temporary disruption of global ocean circulation patterns. Immediately following each Heinrich event were periods of rapid warming known as Dansgaard-Oeschger (D-O) events. These D-O events have been particularly well documented in the north Atlantic Ocean.
In the central GoC, D-O events are associated with abrupt increases in biologic productivity (Figure 1), attesting to a strong link with surface waters conditions in the North Atlantic. Diatom and silicoflagellate assemblages suggest that sea surface temperatures (SSTs) rose in the GoC during the warm D-O events. This implies that transport of monsoonal moisture to the southwest US likely increased during warm D-O events. Cave speleothem isotope records from Arizona and New Mexico have been interpreted to reflect decreased winter precipitation during warm D-O events. Limited precipitation proxy evidence from packrat middens, however, is supportive of increased summer precipitation in the southwest US during some of the warm D-O events.
These results support the sensitivity of the GoC and hence monsoonal rainfall in the southwest US to Pacific SST variations. Future scenarios from climate models call for warmer Pacific SSTs, which may result in conditions similar to the pre-Holocene situation, with different sources and seasonality of precipitation than the modern southwest US.
The paper, published in Marine Micropaleontology in 2014, can be found at: https://pubs.er.usgs.gov/publication/70047920.
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