Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse

Abstract

The Great Ordovician Biodiversification Event (GOBE) was the most rapid and sustained increase in marine Phanerozoic biodiversity. What generated this biotic response across Palaeozoic seascapes is a matter of debate; several intrinsic and extrinsic drivers have been suggested. One is Ordovician climate, which in recent years has undergone a paradigm shift from a text-book example of an extended greenhouse to an interval with transient cooling intervals – at least during the Late Ordovician. Here, we show the first unambiguous evidence for a sudden Mid Ordovician icehouse, comparable in magnitude to the Quaternary glaciations. We further demonstrate the initiation of this icehouse to coincide with the onset of the GOBE. This finding is based on both abiotic and biotic proxies obtained from the most comprehensive geochemical and palaeobiological dataset yet collected through this interval. We argue that the icehouse conditions increased latitudinal and bathymetrical temperature and oxygen gradients initiating an Early Palaeozoic Great Ocean Conveyor Belt. This fuelled the GOBE, as upwelling zones created new ecospace for the primary producers. A subsequent rise in δ13C ratios known as the Middle Darriwilian Isotopic Carbon Excursion (MDICE) may reflect a global response to increased bioproductivity encouraged by the onset of the GOBE.

Glacial onset predated Late Ordovician climate cooling

“…This nonlinear response suggests that the development of a large‐scale ice sheet during the Ordovician could have occurred in conjunction with a moderate tropical sea surface temperatures signal, the amplitude of which would be within the error bars of routine geochemical analyses [e.g., Finnegan et al , ] and thus potentially below the detection limits. This further implies that warm tropical Ordovician seas [ Trotter et al , ; Finnegan et al , ] and geological evidence for glacioeustatic events [ Loi et al , ; Turner et al , ; Dabard et al , ; Rasmussen et al , ] are not mutually exclusive (30.3°C at 12 PAL, Figure ). Interestingly, experiments using a lowered topography (section 3.3.1) are typified by a glacial onset occurring at 8 PAL (instead of 12 PAL).…”

“…It is also still unclear how glacial onset could have possibly occurred in the midst of a prolonged period of hot climate. Evidence for warm shallow seas in the tropics (up to 37°C) [ Finnegan et al , ], as suggested by most recent Ordovician geochemical data [ Trotter et al , ; Finnegan et al , ], is in apparent contradiction with Middle to Upper Ordovician deposits where glacioeustatic events are identified [ Loi et al , ; Turner et al , ; Elrick et al , ; Dabard et al , ; Rasmussen et al , ] or suggested [ Nielsen , ] (see discussion in Amberg et al []). These eustatic cycles support the presence of ice sheets since the Middle Ordovician Darriwilian (467 Ma) rather than a sudden emplacement of a massive ice sheet restricted to the Hirnantian stage (445–444 Ma).…”

RNames, a stratigraphical database designed for the statistical analysis of fossil occurrences – the Ordovician diversification as a case study

“…Correlation coefficients between series of proportional change (Δt/t-1) of subsampled diversity based on stratigraphic opinions with one-time bin versus two-time bin minimum resolution. Generalised differencing was used to reduce effects of autocorrelations (McKinney, 1990;Alroy, 2000;Novack-Gottshall and Miller, 2003 during the Darriwilian (Rasmussen et al 2016;Trubovitz and Stigall, 2016) …”

Permian strontium isotope stratigraphy

“…JLs-1 for carbonates: Rasmussen et al 2016). It is further encouraged to analyse element concentrations from a split of each sample solution for preservation control (Ullmann et al 2013(Ullmann et al , 2014Rasmussen et al 2016). …”