Holocene geochronology of a continental-shelf mudbelt off southwestern Africa

被引:25
作者
Meadows, ME [1 ]
Rogers, J
Lee-Thorp, JA
Bateman, MD
Dingle, RV
机构
[1] Univ Cape Town, Dept Geog & Environm Sci, ZA-7701 Rondebosch, South Africa
[2] Univ Cape Town, Quaternary Res Ctr, ZA-7701 Rondebosch, South Africa
[3] Univ Cape Town, Dept Geol Sci, ZA-7701 Rondebosch, South Africa
[4] Univ Cape Town, Dept Archaeol, ZA-7701 Rondebosch, South Africa
[5] Univ Sheffield, Dept Geog, Sheffield Ctr Int Drylands Res, Sheffield S10 2TN, S Yorkshire, England
[6] Inst Geol, DK-1350 Copenhagen, Denmark
关键词
mudbelt; continental shelf; geochronology; Holocene; sedimentation patterns; southern Africa;
D O I
10.1191/0959683602hl521rp
中图分类号
P9 [自然地理学];
学科分类号
0705 ; 070501 ;
摘要
The Holocene lithostratigraphy and geochronology of sediments on an inner continental-shelf mudbelt off the west coast of southern Africa is presented. Based on a study of seven large gravity cores, numerous grab samples and two vibracores, two distinctive sections of the mudbelt have been identified. The northern section is associated with the Orange River prodelta, and is dominated by laminated clay-rich sediments, while the southern section off Namaqualand is associated with more homogeneous muds. AMS radiocarbon ages of 31 samples from these cores are problematic due to the absence of expected modern material at the sediment-ocean interface and numerous age reversals and inconsistencies in some of the cores. Possible mechanisms to explain the apparently anomalously old surface and near-surface samples are suggested. Most likely explanations appear to lie in a combination of sediment-retrieval problems, the periodic loss of fine particulate organic carbon from within the water column, the sampling of possible relic sediments and, most likely of all, the incorporation into the sediments of C-14-depleted terrigenous organic matter. The age reversals are especially evident in the laminated sediments, and the entrainment of 'old' carbon from the terrestrial source of these sediments is offered as the most parsimonious explanation. Stable carbon isotope analyses and a single luminescence age support the contention that radiocarbon date inconsistencies are a consequence of terrestrial inputs of carbon which is non-contemporaneous with the actual time of sedimentation. In studies of offshore accumulations of terrigenous material, the marine-derived organic fraction may therefore prove a more reliable measure of radiocarbon chronology.
引用
收藏
页码:59 / 67
页数:9
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