Search

The geological history of the South Asian monsoon (SAM) before the Pleistocene is not well‐constrained, primarily due to a lack of available continuous sediment archives. Previous studies have noted an intensification of SAM precipitation and atmospheric circulation during the middle Miocene (∼14 Ma), but no records are available to test how the monsoon changed prior to this. In order to improve our understanding of monsoonal evolution, geochemical and sedimentological data were generated for the Oligocene‐early Miocene (30–20 Ma) from Indian National Gas Hydrate Expedition 01 Site NGHP‐01‐01A in the eastern Arabian Sea, at 2,674 m water depth. We find the initial glaciation phase (23.7–23.0 Ma) of the Oligocene‐Miocene transition (OMT) to be associated with an increase in water column ventilation and water mass mixing, suggesting an increase in winter monsoon type atmospheric circulation, possibly driven by a relative southward shift of the intertropical convergence zone. During the latter part of the OMT, or “deglaciation” phase (23.0–22.7 Ma), a long‐term decrease in Mn (suggestive of deoxygenation), increase in Ti/Ca and dissolution of the biogenic carbonate fraction suggest an intensification of a proto‐summer SAM system, characterized by the formation of an oxygen minimum zone in the eastern Arabian Sea and a relative increase of terrigenous material delivered by runoff to the site. With no evidence at this site for an active SAM prior to the OMT we suggest that changes in orbital parameters, as well as possibly changing Tethyan/Himalayan tectonics, caused this step change in the proto‐monsoon system at this intermediate‐depth site. Geochemical data from eastern Arabian Sea indicate Oligocene‐Miocene transition is key interval of strengthening of South Asian proto‐monsoonEarly phase of transition (23.7–23.0 Ma) displays intensification of winter monsoon‐like circulation associated with global cooling“Deglaciation” phase (23.0–22.7 Ma) triggers the establishment of OMZ, signaling the strengthening of proto‐monsoon atmospheric circulation Geochemical data from eastern Arabian Sea indicate Oligocene‐Miocene transition is key interval of strengthening of South Asian proto‐monsoon Early phase of transition (23.7–23.0 Ma) displays intensification of winter monsoon‐like circulation associated with global cooling “Deglaciation” phase (23.0–22.7 Ma) triggers the establishment of OMZ, signaling the strengthening of proto‐monsoon atmospheric circulation

Stratigraphic data relating to the Aptian and Albian sediments around Folkestone in southeast England, on the eastern margin of the Wessex Basin, are plotted graphically to illustrate biostratigraphic gaps and condensed intervals. These are then used to suggest candidate sequence boundaries, notated LG1-4 and G1-3, which are named after the Lower Greensand and Gault. The biostratigraphic subzones apparently unrepresented in the area are: LG1 obsoletus and bodei; LG2 gracile and debile; LG3 schrammeni; G1 bulliensis. Candidate sequence boundaries G2 (cristatum Subzone) and G3 (auritus Subzone) are erosion surfaces correlating with probable biostratigraphic gaps elsewhere in the basin. The age of LG4 is uncertain but must lie within the mammillatum Superzone. All candidate sequence boundaries correspond closely to those predicted by recent global sea-level cycle charts, and additionally there is a close match between predicted maximum flooding surfaces and the occurrence of condensed intervals. Most of the candidate sequence boundaries could have been formed as a result of rapid, and at least regionally extensive, relative sea-level falls, but the sedimentological contexts of LG3 and G1 suggest that these may have been generated by transgression-related sediment condensation. The minor sequence boundary in the loricatus Zone (intermedius Subzone) illustrated in the cycle charts cannot be unambiguously recognized at outcrop in the Folkestone area. Additional sequence boundaries not shown in the cycle charts may also be present in the Wessex Basin.

The transition from evaporitic and lagoonal Purbeck Beds into the fluviatile basal Wealden Beds (Ryazanian-Valanginian) has in the past been interpreted as a continuous regressive succession. Mupe Bay and Bacon Hole, in south Dorset, are the only localities in the Wessex Basin of southern England at which the transition strata are well exposed and not extensively faulted. On the basis of facies analysis the lowermost Wealden Beds are interpreted as a regressive-transgressive-regressive package. Environments fluctuated between lagoon, inner-lagoonal shoreline and fluvial flood plain. The overlying succession is remarkable for the presence of two major erosion surfaces which, based on field geometries and facies relationships, are interpreted as unconformities. The lower unconformity is overlain at Mupe Bay by sediments of lagoonal or lacustrine origin. The upper unconformity is expressed as the celebrated Mupe Bay palaeo-oilseep, an oil-cemented conglomeratic sand, overlain by a thick succession of fluvial sediments. A level near the basal sand and the two erosion surfaces are treated as candidate sequence boundaries (WB1-WB3), with the thin intervening ?lagoonal deposits representing peaks of transgression. The pattern of one minor (conformable) sequence boundary followed by two major (unconformable) sequence boundaries bears a strong resemblance to the Early Cretaceous sequence stratigraphy suggested in recent 'global cycle charts'. However, biostratigraphical calibration of the sections is currently inadequate to allow close correlation with candidate sequence boundaries in other areas.