Sea-level curve for the Middle to early Late Ordovician in the Armorican Massif (western France): Icehouse third-order glacio-eustatic cycles

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).…”

“…The period of the sinusoidal variation in obliquity is set to 30 kyr, based on spectral analysis of Ordovician halite deposits [ Williams , ]. Orbital variations are known as a major driver of Pleistocene glacial cycles [e.g., Paillard , ], and they have been shown to significantly impact the growth of the ice sheet during the Ordovician as well [ Herrmann et al , ; Ghienne et al , ; Dabard et al , ]. An ice sheet of larger extent is simulated, provided that climate is favorable to ice sheet growth: in order to account for the feedbacks of the ice sheet on climate, the largest ice sheet simulated previously is reintroduced as a boundary condition into the climate model (still keeping the CO 2 constant). Two additional glacial climatic states are simulated, one for each of the two opposite orbital configurations (CSO and HSO).…”

“…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).…”

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

“…9 Myr (Cohen et al, 2013). Comparable Middle Ordovician stratigraphic sections in the Armorican Massif of France, for example, consist of several hundred meters of strata (Dabard et al, 2015).…”

“…This hypothesis could contradict the concept of long-term global greenhouse climate stability during most of the Ordovician, before the cooling interval of the Late Ordovician. Arguments of a glacial onset that may have occurred as early as the Darriwilian at the Gondwanan paleoSouth pole, predating the Hirnantian glacial maximum , are sedimentological, geochemical, micropaleontological or related to the paleogeographical configuration (Dabard et al, 2015;Thompson, 2011;Trotter et al, 2008;Turner et al, 2012;Pohl et al, 2014;Pohl et al, 2016;Rasmussen et al, 2016;Vandenbroucke et al, 2010a, b, etc.). Since the present contribution deals with the palynology of Gondwanan rocks of ?Dapingian to Darriwillian age, it is interesting to consider whether palynomorph assemblages from borehole QSIM-801 have any bearing on the hypothesis of precocious cooling in the Middle Ordovician.…”