Orbital forcing of the late Pleistocene boreal summer monsoon: Links to North Atlantic cold events and El Nino; Southern Oscillation. Geologica Ultraiectina (313)

Abstract

This thesis revolves about the timing of precession-related variations in the boreal summer monsoon and the impact of North Atlantic cold events and the El Nino Southern Oscillation on this timing. Transient climate modelling experiments indicate that the intensity of the Northern Hemisphere summer monsoon varies in-phase with peak summer insolation on orbital timescales. In contrast, marine proxy records suggest a lagged response of the monsoon maximum of a few thousand years behind the insolation forcing. This phase lag is particularly large in records from the Arabian Sea. In this thesis it is shown that a ~3,000 year lag of the African monsoon, which is derived from the Mediterranean sapropel record, is consistent not only with the marine isotope chronology but also with radiometrically dated speleothem records from the Mediterranean region and China, and with the atmospheric methane record from Antarctica. It is suggested that North Atlantic cold events systematically delay the onset of strong monsoon intensity. This explanation implies a distinct precession forcing of North Atlantic ice rafting and could also explain a non-stationary behaviour of the African Monsoon over the past 3 million years. An orbital-tuning independent chronology for a North Atlantic foraminiferal benthic oxygen isotope stacked record over the last two glacial cycles supports this theory and suggests that North Atlantic ice surge events respond to an external, climatic triggering mechanism, which is distinct from the process that causes the waxing and waning of the large ice sheets. A comparison with a climate modelling study of the El Nino Southern Oscillation on orbital time scales shows that the ice surge events coincide with periods of frequent El Nino events. Warming of northern North America during El Nino events could have led to rapid disintegrations of fringing ice shelves induced by meltwater infilling of surface crevasses, thereby triggering large-scale ice surge events. It is moreover suggested, that the long phase-lag that is derived from Arabian Sea productivity records is related to a decoupling of the Asian summer monsoon and Arabian Sea productivity at the precession frequency band. The intensity of the meridional overturning in the Atlantic potentially dominates the precession signal in the Arabian Sea. According to ocean modelling, increased Atlantic overturning results in an increased nutrient delivery into the euphotic layer of the northern Indian Ocean and triggers thereby high productivity and oxygen minimum zone maxima. The same process is suggested as explanation for anomalously high productivity conditions during marine isotope stage 13. To the end of the thesis a sea surface temperature record from the Gulf of Mexico over the last 300,000 years is presented. Variations in this record are controlled by the migration of the northern boundary of the Atlantic Warm Pool and are relatively insensitive to winter conditions. Because a clear signature of rapid climate-change events, such as the Younger Dryas cold event, is lacking in the record, it is concluded that high-latitude cold events influence only the winter Caribbean climate conditions, supporting the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events.