Mineralogy, geochemistry and microfacies of late Quaternary periplatform sediments: Carbonate export cycles and secondary processes - Sanganeb Atoll and Abington Reef, Sudan, Central Red Sea Mineralogie, Geochemie und Mikrofazies spätquartärer Periplattformsedimente: Karbonatexportzyklen und sekundäre Prozesse - Sanganeb Atoll und Abington Riff, Sudan, Mittleres Rotes Meer Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Peter Emmermann Kiel 2000 Referent/in: .......................................................................... Koreferent/in: .......................................................................... Tag der mündlichen Prüfung .......................................................................... Zum Druck genehmigt: Kiel, den .......................................................................... Der Dekan .......................................................................... ABSTRACT Variations in carbonate production and sediment export of Sudanese off-shore reefs were studied in response to late Quaternary eustatic sealevel changes in the Red Sea. A set of sediment cores obtained from the Sudanese shelf in the vicinity of Sanganeb Atoll and Abington Reef was analysed for glacial-interglacial variations of shallow-water input in the periplatform sediments that enclose the off-shore reefs in a present-day waterdepth of about 500 to 800 meters. The periplatform record reaches back to marine isotope stages 4 to 6 and all cores show the last glacial-interglacial cycle. Aragonite/calcite-ratios and strontium content of the periplatform sediments were analysed being excellent proxies for shallow-water export variations. In addition, the grainsize and component distribution patterns of the periplatform sediments were analysed and compared to the mineralogical data in order to separate sediment input variations from secondary processes like dissolution and precipitation of metastable carbonates at the seafloor. The periplatform sediments are rather uniform and consist of a greenish-grey, quartz- and bio-detritic, foraminifer- and pteropod-bearing, nanno-ooze, which in proximal cores contains shal- low-water calciturbidites. During the last glacial marine isotope stage 2 (IS 2) and the following deglaciation this stand- ard type of periplatform sedimentation was interrupted by submarine lithification (23,000-13,000 14C-years) and sapropel formation (13,000-8,500 14C-years). The last glacial eustatic sealevel lowstand caused a restriction of water-mass exchange between the Indian Ocean and the Red Sea. Accompanied by high evaporation rates during a predominantly arid climate (NE-monsoon) the salinities of Red Sea surface- and bottom-waters rose to values over 50‰. These high salinities restricted marine biota and led to scarcity or absence of planktic foraminifers and coral reefs van- ished from the Red Sea. Diminished biogenic carbonate production was replaced by inorganic carbonate precipitation which was favoured by high salinities and reduced pelagic and shallow- water input. Stable oxygen isotopes and inorganic aragonite precipitation reached a maximum that coincides with highest bottom-water salinities of 57.5‰ at 14,480±110 14C-AMS years. This maxi- mum coincided with the eustatic sealevel lowstand in the Red Sea and the Western Indian Ocean at about 15,000 14C-years, which corresponds to approximately 18,000 calendar years. During the pase of submarine lithification the precipitation mode switched from an early phase of Mg-calcite and aragonite precipitation to a predominance of aragonite since 19,540±130 14C-AMS years, which points to progressive basin restriction in phase with sealevel lowering. The frequent alteration of lithified and unlithified layers within the lithified sequence was caused by terrigenous input variations. In the non-lithified layers a higher detritic input prevented carbonate precipitation and cementation. Sapropel formation took place between 13,000 and 8,500 14C-years on the Sudanese shelf and terminated the lithification process and the hypersaline conditions of bottom- and surface- waters as shown by a significant decrease in oxygen isotope values of the lithified sediments and the sudden occurrence of planktic foraminifers. The deglaciation period in the Red Sea is marked I by a pluvial phase which led to stagnation of bottom-waters and a higher input of organic matter of terrestrial sources from the Sudanese hinterland, which is clearly recorded by an increased quartz content. The development of a pycnocline and the high input of organic matter caused oxygen depletion which led to the formation of a sapropel and increased aragonite preservation as shown by the unusual preservation spike in the periplatform record between 13,000 and 8,500 14C-years. Except for the previously breaks in periplatform sedimentation it was shown to some extend that the overall late Quaternary shallow-water sediment export pattern varies in tune with glacio- eustatic sealevel variations as proposed by the highstand shedding theory (Schlager & James, 1978; Schlager et al., 1994). Variations of aragonite/calcite-ratios and strontium-content in the periplat- form sediments record the shallow-water export variations of the reefs but the signal is obscured by post- and syndepositional processes in concert with climatic and hydrologic anomalies of the nearly isolated Red Sea basin. However, for the Holocene sealevel rise it was shown that highest shallow-water export is in phase with the flooding of the old Pleistocene reef structures at about 8,000 years BP. The signifi- cant increase in the accumulation rates of aragonite and strontium mark the onset of prolific reef growth and sediment export close to the end of the Holocene sealevel rise. This is also confirmed by a significant increase of the total reef growth area calculated for Sanganeb Atoll after the flood- ing of the Pleistocene reef surfaces which lie in 20 to 25 meter below present sealevel (mbps). The simulation of flooded reef areas at Sanganeb Atoll also showed that flooding and exposure of the inner lagoon, which reaches down to a present-day waterdepth up to 50 mbps has an important influence on carbonate production, whereas reef growth on submarine terraces has no significant impact on the total carbonate export because the areas are too small to produce significant amounts of sediment when compared to the total reef area. A generally increased shallow-water sediment export during the Holocene is also shown by bulk sedimentation rates which are 1.5 to 2 times higher than glacial values, which is in the range of glacial-interglacial sediment export variations in the Bahamas. Glacial-interglacial sediment-export variations in the Sudanese periplatform sediments are also shown by grainsize and component distribution patterns. During the Holocene the input of fine-grained aragonite dominates while during the glacial isotope stage 3 (IS 3) more coarse grained shallow-water components are found in the periplatform sediments. This points to (1) prolific reef growth and sediment export during IS 3 at Sanganeb Atoll and (2) to a glacial-interglacial shift in shallow-water carbonate production. During the Holocene sealevel highstand more fine grained aragonite was produced in the lagoon, while during glacial lowstands (IS 3) benthic carbonate production was limited to the outer slopes when the inner platform became exposed resulting in the export of relative coarse grained sediment. Although coral reefs vanished from the Red Sea during marine isotope stage 2 (IS 2) the frequency of shallow-water derived calciturbidites was high during this phase as well as during IS 3. Radiocarbon dated scleractinian fragments of the turbidites are about 5,000 to 6,000 years older when compared to the stratigraphic position of the turbidite within the periplatform sequence. The stratigraphic position of the calciturbidites shows that older shallow-water sediments were re- II sedimented during the sealevel fall between isotopic event 3.3 (53,000 SPECMAP-years) and the sealevel lowstand at 14,840±110 14C-AMS years. Thus, the ages of the scleractinian fragments give evidence for shallow-water carbonate production and reef growth during IS 3 and possible early IS 2 up to 21,480±180 14C-AMS years at Sanganeb Atoll. Furthermore it shows, that a high frequency of calciturbidites is also found during sealevel fall and lowstands and that „highstand bundling“ as found in the Bahamas is not the case at Sanganeb Atoll. As mentioned above the glacial-interglacial sediment export pattern in the Red Sea is ob- scured by post- and syn-depositional processes. Better preservation of aragonite as observed dur- ing the pluvial phase of the last deglaciation, which led to sapropel formation on the Sudanese shelf is also found during IS 3. Peaks in the aragonite and TOC curve coincide with decreased δ18O- values and indicate short-termed preservation events in concert with monsoonal climate variations. In analogy to the Arabian Sea it was shown for the Red Sea that those variations are connected to high-latitude temperature oscillations. More humid phases (SW-monsoon) led to better aragonite preservation in analogy to the situation during the deglaciation. The aragonite/calcite-ratios of the periplatform sediments are clearly altered by inorganic precipitation of aragonite during IS 2 and Mg-calcite during IS 4. Inorganic precipitation of Mg- calcite during IS 4 was