Orbitally-induced changes of the Atlantic and Indian monsoons over the past 20,000 years: New insights based on the comparison of continental and marine records

Abstract

International audienceVariations of Atlantic and Indian monsoon systems since the last glacial period are investigated by comparing eolian fluxes from two marine cores (ODP site 658 off western Africa and core 74KL off the Arabian peninsula) with 2147 hydrological records (lacustrine, palustrine, spring and fluvial, arid) gathered over a large continental area extending between 10 and 30oN across Africa, Arabia and western India. We show that the hydrological response to the Holocene humid phase in the northern tropics strongly differs from a region to another. The humid period is significantly shorter in the Arabian peninsula and the horn of Africa compared to northern Africa even though its maximum is contemporaneous (11,000-7,000 cal yr BP). Western India displays a specific hydrological signal characterized by the importance of well-developed fluvial systems from the Himalayas and the paucity of lakes compared to the other regions. In western India, the humid peak is shifted toward the mid Holocene (8,000-6,000 cal yr BP). Both marine records show a peak between ~ 11,000 and 7,000 cal yr BP for the Holocene humid period, in good accordance with African-Arabian records. However, while continental hydrological data suggest that the onset and termination of this humid period might have been relatively progressive, the marine windborne records indicate abrupt transitions, somewhat out-of-phase with continental evidence (e.g. abrupt decrease of aeolian proxies as early as ~ 15,000 cal yr BP). Discrepancies between marine and continental likely result from the fact that aeolian fluxes at a given marine location do not simply record monsoon-related changes of humidity over the adjacent continental sources but could be affected also by changes of the source area (e.g., emersion of the Arabo-Persian gulf associated to the glacial, low sea-level stand), and changes in wind intensity and/or direction

Hydro-climate changes over southwestern Arabia and the Horn of Africa during the last glacial–interglacial transition: A pollen record from the Gulf of Aden

“…But the end of the HHP, on the other hand, was rather progressive (Foerster et al, 2012), suggesting a climatic transition that was gradual, confirming what has been deduced from lacustrine sediments in the Sahara desert (Kröpelin et al, 2008). Lézine et al (2014) stressed out regional differences in the dynamics of HHP onset and termination based on a comprehensive dataset of nearly 2147 well-dated hydrological records (lacustrine, palustrine, spring and fluvial, arid). They showed that the Arabian Peninsula and the Horn of Africa have experienced a significantly shorter humid period compared to northern Africa even though the optimum of the Holocene humiditywas contemporaneous.…”

“…Foerster et al, 2012;Tierney and deMenocal, 2013; Figure 6E) and speleothems from Socotra (Shakun et al, 2007;Van Rampelbergh et al, 2013) which date the onset of the post glacial humidity increase from 14.68-14.5 ka ( Figure 6D). Such an early increase in humidity is also evidenced in the development of lakes and wetlands over the Horn of Africa (Lézine et al, 2010(Lézine et al, , 2014; Figure 6F), although the signal remains weak.The trend of humidityin the pollen record from core MD92-1002 was punctuated by two phases of return to dryer conditions: at 12.7ka, then at 10.4ka, in phase with periods of enhanced wind activity of N-S direction responsible for dust…”

“…The exceptionallyhigh sedimentation rates, which characterize this record (~45cm/ka), enable us to cope in details with the structure and timing of the last deglaciation. Comparison with earlier studies from marine cores (Core P178-15P - Tierney anddeMenocal, 2013, andcore KL74 -Sirocko et al, 1993) and continental archives from both lacustrine deposits (Lézine et al, 2014) and speleothems (Neff et al, 2001;Fleitmann et al, 2007;Shakun et al, 2007;Van Rampelbergh et al, 2013) make it possible toexamine the relative influence of southern (African) or northern (Arabian) origin and reconstruct the spatial and temporal changes of summer monsoon, with a special emphasis on the Holocene humid period and environmental changes across the last deglaciation.…”

“…They showed that the Arabian Peninsula and the Horn of Africa have experienced a significantly shorter humid period compared to northern Africa even though the optimum of the Holocene humiditywas contemporaneous. Lézine et al (2014) also concluded that the onset and termination of the HHPwereprogressive in western Africa, but more abrupt in eastern Africa (Sudan, Egypt), and possibly also over the Horn of Africa and Arabia, although the low amount ofwell-dated hydrological data in these two regions made it more difficult to conclude.…”

“…Maxima of %dolomite reached during the last glacial are at least 3 timeshigher (or more) than those of the late Holocene. Even though the last glacial maxima of KL74 dolomite may have been amplified by terrestrial inputs from large dessicated areas such as the Persian Gulf (Sirocko et al, 1993;Lézine et al, 2014), pollen record from core MD92-1002 suggests that wind activity and correlative dryness werehigher during the last glacial than during the Holocene humid period. This pattern is also evidenced by theD wax record from nearby core P178-15P,which never falls down to glacial values during the late Holocene (Tierney and deMenocal, 2013; Figure 6E).…”