Geosphere and biosphere dynamics during late Ordovician climate change

Abstract

The late Ordovician was a period of major climatic and biological change, much of which is poorly understood. Global cooling began in the Caradoc (early Katian) with the build-up of ice in southern polar regions of the palaeocontinent of Gondwana. Cooling continued into the Ashgill (late Katian) but may have been interrupted by a brief period of global warming, the Boda Event, in the Cautleyan-Rawtheyan immediately prior to the Hirnantian glacial maximum. The proceeding Hirnantian Stage of the Upper Ordovician was a period of abrupt global change in the biosphere, climate and ocean geochemistry. These events are marked by: (1) an abrupt positive Hirnantian isotopie carbon excursion (HICE); (2) one of the three global Phanerozoic mass extinctions and (3) an extensive drop in sealevel associated with the maximum extent of the Gondwanan ice sheet. Biostratigraphic correlation for the Late Ordovician between basin and shelf sections in the Welsh Basin is limited. The current study describes three new chitinozoan taxa, Spinachitina penbryniensis, Belonechitina reticulatus and Belonechitina ceredigionensis and developed a chitinozan biostratigraphic scheme for the Welsh Basin. Four of the six Avalonian Ashgill chitinozoan biozones are recognized: the bergstroemi fossensis, umbilicata and taugourdeaui Biozones. The Baltoscandian and Laurentian index taxon Hercochitina gamachiana is recorded for the first time in Avalonia and a new lower Hirnantian regional biozone, the new Belonechitina reticulatus n. sp. Biozone is erected. The Cautleyan-Rawtheyan (late Katian) rugata Biozone was not recorded. Four depth-facies biotopes for chitinozoa from the upper Katian-Hirnantian (Upper Ordovician) of Avalonia have been identified herein. These are: 1) an open ocean shallow-water epipelagic biotope which includes Cyathochitina campanulaeformis; 2) an open ocean middle-depth mesopelagic biotope comprising Hercochitina and Spinachitina; 3) an open ocean deep-water meso-bathypelagic biotope characterized by Bursachitina umbilicata and 4) a shelf biotope containing predominantly Desmochitina. The taxonomic composition of the open ocean middle- depth mesopelagic biotope changes from one dominated by Hercochitina and Spinachitina in the Upper Katian to one comprising Ancyrochitina in the Hirnantian post-glacial transgression. The distribution of the deep-water meso- bathypelagic biotope and the shelf biotope is affected by basin hydrography and sea level respectively. Changing hydrography induced by climate belt reconfiguration along the southern margin of lapetus in the Rawthean (late Katian), displaced deep-water taxa into the semi-restricted Welsh Basin. With sea level fall in the early Hirnantian, taxa from the shelf biotope expanded into the shallower basin. Origination of biostratigraphically useful taxa in such segregated environments, e.g. Bursachitina umbilicata, restricts their distribution to periods of climatic and environmental change. Gradual change in climate and, hence, distribution of important biozone taxa, leads to diachroneity of chitinozoan bio-zones. During the Katian and Hirnantian, the Welsh Basin, UK, lay on the northern margin of the palaeocontinent of Avalonia at mid-latitudes (estimated between 32-45 s), within the boundaries of the present-day position of the STHP. It therefore provides a suitable environment to test for sensitivity of palaeo-climate belt movement in a mid-palaeolatitude setting (Armstrong et al, in revision).When the Inter-Tropical Convergence Zone (ITCZ) and Sub-Tropical high pressure belt (STHP) were in a southerly position, mid-palaeolatitudes, which includes the Welsh Basin, were positioned beneath south-easterly trade winds. This climate belt configuration is recorded in the Welsh Basin by the deposition of organic-rich laminated hemipelagites deposited during wind-driven coastal up-welling. Coastal upwelling, increased productivity and organic carbon burial are recorded by increased Ba/Th, increased TOC wt% and negative δ(^13)C. Negative δ (^13)C excursions are interpreted to represent remobilization of (^12)C֊enriched waters during upwelling from the deep-ocean. The onset of the expression of climate belt movement during Boda Event times is later in the Welsh Basin than at low-palaeolatitudes. At low-palaeolatitudes the δ(^18)Ο response to shifting ITCZ occurred in the Cautleyan whilst in the Welsh Basin the isotopic and lithological response to moving STHP began in the Rawtheyan. This delay in expression of events is accounted for by ice-sheet- moderated rectification of insolation in the run-up to the glacial maximum. The onset of the Hirnantian glacial maximum is orbitally-moderated and is interpreted to represent an orbital configuration when eccentricity wag high, obliquity low and aphelion occurred during Austral summer. It represents a threshold at which the Gondwanan ice-sheet was able to survive precession and obliquity-induced ablation. Once this threshold had been reached, global climate belt position stabilized in a position where the ITCZ lay north of the equator and the STHP lay north of Avalonia. Four positive δ (^13)Corp excursions in Hirnantian-age rocks from the Welsh Basin have been recognized in this study and are considered to represent the HICE. They correlate well between basin and shelf environments and also with a proposed sequence stratigraphic model for the Hirnantian in the Welsh Basin. Two excursions of up to 3 %(_00) occur in the lower Hirnantian reticulatus chitinozoan Bio- zone and two excursions of 1 %(_00) are present in the late Hirnantian taugourdeaui Biozone. Higher carbon storage in the basin, coincident with positive δ (^13)Corp excursions during glacial periods, indicate a shift in the locus of carbon burial from the shelf to the basin during low sealevel. The topology of the regional Hirnantian isotope curve for the Welsh Basin compares with only a few curves from other parts of the world indicating strong regional carbon cycling. Comparison of the relative timing of the onset of HICE and peak HICE values from other palaeocontinents demonstrate little, if any, consistency with palaeolatitude or basin setting. It is demonstrated that this chemostratigraphic diachroneity of HICE can be attributed to the process of signal rectification of δ (^13)Corp records most likely due to numerous local carbon cycling processes.