NEOPROTEROZOIC ICE-RAFTING EVENTS RECORDING A ‘CONVENTIONAL’ HIGH-LATITUDE GLACIATION? IMPLICATIONS FOR PALAEOCLIMATE MODELS

Five discrete ice-rafted debris (IRD) intervals occur within the Southern Highland Group of the Dalradian Supergroup, Donegal, Ireland. These intervals are separated from one another by many tens of metres of heterolithic strata, which importantly include many tens of centimetres of hemipelagic sediments that are devoid of IRD. We interpret this succession as recording a series of (at least) five ice-rafting events. As such, this Neoporoterozoic glaciomarine succession records deposition characterised by the existence of contemporaneous terrestrial wet-based ice-sheets, open seas and a fully functioning hydrological cycle (high-latitude snowfall necessary to recharge ice-sheets). Sedimentological analysis utilising modern glaciomarine sedimentation rates indicate that these events occurred with a quasi-millennial-scale periodicity.

The Culdaff Limestone occurs ca. 100 m below the base of the glaciomarine succession. Carbonate d¹³C values for this unit exhibit a decline from positive (+2ppm PDB) to negative (-3.5ppm PDB) over ~40 metres, a decline which mimics those inferred for snowball Earth scenarios. This would imply that the overlying glaciomarine succession would represent either snowball maxima or meltback. However, the occurrence of quasi-periodic ice-rafting events with their associated environmental parameters is incompatible with those inferred in the snowball Earth hypothesis. In addition, palaeomagnetic constraints (Catoctin and Callander poles) indicate a high latitudinal position for eastern Laurentia at ca. 580-565 Ma, a time which spans the age of the lower Southern Highland Group glaciomarine succession. Thus, such an occurrence similarly negates the high-obliquity model for this late Neoproterozoic glaciation.