Controls on carbonate and terrigenous deposition in the incipient Benguela upwelling system during the middle to the late Miocene (ODP Sites 1085 and 1087)

Abstract

This study deals with the variability of biogenic carbonate and terrigenous depositional patterns in the Cape Basin area of the South Atlantic during the middle to the late Miocene focusing on ODP Sites 1085 and 1087 sediments at the Benguela Upwelling area, Southwest Africa. The investigations are based on grain-size analysis and clay mineralogy. The sedimentary records at both sites provide information about climate and environmental changes. The direct input of terrigenous material from southwestern Africa is driven by a complex interaction of river and wind transport, and ocean currents. The considerable amount of dust input derived from the Namib Desert started to become important after 11.2 Ma, pointing to an increased supply of cool intermediate waters to mid-latitude surface waters in the upwelling regions, which coincided with the establishment of the winddriven Benguela upwelling system. A major drop in CaCO 3 concentration between 10.4 and 10.1 Ma is caused mainly by changes in calcareous nannoplankton production, while another drop between 9.6 and 9 Ma is triggered by a combination of production changes of calcareous nannoplankton and dilution, in response to high shelf supply during global lowering of the sea level.

The middle to late Miocene climatic development of Southwest Africa derived from the sedimentological record of ODP Site 1085A

“…6c). This rise is consistent with the findings of Kastanja et al (2006), who also found an enhanced MAR TOC from 11.8 Myr on. It also coincides in time with the rise of MAR bulk .…”

“…This study incorporates and expands the results and findings of earlier studies by Kastanja et al (2006) who showed, that dust input from the Namib desert started to become identificable after 11.2 Myr. This gives indications for the establishment of a major shift in the wind regime, which has been a main precondition for the development of the wind-driven Benguela Upwelling System (BUS).…”

“…At Arrisdrift fossil ages range from 18 to 12 Myr (Partridge 1993). The records of the climatic development of SW-Africa and the SE Atlantic show that the upwelling in the Benguela region starts about 10 Myr ago (Kastanja et al 2006).…”

“…From this interval 30 samples were taken with a resolution of about 100 kyr beginning at 13,891 kyr up to 10,975 kyr. Also data from sections 47X-55X (457.36-534.26 m), which were collected in higher resolution by Kastanja et al (2006) were used for this study. This second interval comprises 118 samples from 10,967 to 9,006 kyr.…”

“…North Atlantic Deep Water (NADW) did not intrude into the Cape Basin before 10 Myr (Diester-Haass et al 2004). During the Middle Miocene the Benguela Current (BC) progressed northwards and reached the Walvis Ridge at about 10 Myr (Diester-Haass et al 1990Krammer et al 2006;Kastanja et al 2006).…”