Late Quaternary change in the North American (Mexican) Monsoon: variability in terrestrial and marine records and possible mechanisms

The Late Quaternary history of the North American (or Mexican) monsoon (NAM) remains poorly understood, with continuing debates about the relative importance of insolation forcing, the role of the Laurentide Ice Sheet (LIS), the expression of warm (D-O) and cold (H) events in the North Atlantic and the influence of the Pacific. To date, more information has been available from the southern and northern margins of the NAM region than from its tropical and subtropical core. This is significant because to the south of the NAM region, the direct effect of ITCZ location is likely to be stronger and any potential influence of the LIS weaker, and to the north, there is an important change in present day precipitation seasonality (from summer to winter), an opposite response to forcings such as ENSO/PDO and AMO and probably a stronger influence of the LIS. As a result, the interpretation of speleothem records from New Mexico (e.g. Asmerom et al., 2010) and Arizona (e.g. Wagner et al., 2010), in the southwestern USA and marine records such as Cariaco (Peterson and Haug, 2006) and lake records such as Peten Iztá (Hodell et al., 2008) may not be applicable to the tropical NAM core. Here we present results from two lacustrine sequences in Mexico (Sayula 20oN; Babicora 29oN) and a marine core record from the central part of the Gulf of California (27oN) all extending back at least through MIS3 (ca. 60 kyr BP). Although lacking the chronological precision of the speleothem sequences, these multiproxy records preserve evidence of centennial and millennial scale variability. MIS3 is marked by generally wetter conditions in the lake basins and warmer SSTs in the marine record, particularly during D/O events, which can be attributed to a stronger monsoon as well northward displacement of the ITCZ. This contrasts with the standard interpretation of the speleothem sequences where D/O events are dry. In contrast, H events are usually drier/cooler (weaker NAM, reduced summer precipitation), except for H1 which is more variable and where conditions may have been more affected by the LIS. In our records the YD is also drier/cooler, again contrasting with the interpretation of the speleothem sequences. The interpretation of conditions around the LGM is complex, with both lake records indicating more available moisture than today, but whether this reflects reduced evaporation or increased winter precipitation driven by the effects of the LIS is unclear. The extent to which the NAM persisted during the LGM is also an area of continuing debate. The response of the lake sites to the early Holocene insolation maximum is rather muted supporting the view that the persistence of the residual LIS limited any significant strengthening of the NAM, even with a clear northward displacement of the ITCZ and strong warming in the central Gulf of California, especially after about 8 kyr BP. This limited early Holocene enhancement of the NAM is in marked contrast to other northern hemisphere monsoon systems.