Farfield effects of Namurian glaciation

Figure 1. Stable-isotopic ratios of brachiopods from the Woodland borehole suggest a Ω18O seawater (w) value of around –1.8‰ VSMOW and point to an absence of widespread ice-caps. Click to download.

Sedimentary sequences in northern Britain dating from the early Namurian (Carboniferous) record the effects of distant glaciation on palaeoequatorial systems.

Early Namurian glaciation

During the Serpukhovian (early Namurian), glacial conditions were initiated in the southern hemisphere. Evidence for the duration of glaciation is somewhat contradictory. In parts of eastern Australia, the sedimentary record points to an early Serpukhovian onset followed by a long interglacial period. In South America and Tibet, evidence suggests instead that the glaciation lasted throughout the Serpukhovian. New farfield data from early Namurian sediment sequences in boreholes from near Newcastle-upon-Tyne, northern Britain, provide new insight into the apparent discrepancy.

Evidence from the British sedimentary record

Stable-isotopic data have been produced from well-preserved brachiopods and organic matter from three boreholes (the Woodland, Throckley and Rowlands Gill boreholes) in the Yoredale facies deposits of north-east Britain. Signatures of carbonate from late Serpukhovian brachiopods in the Woodland borehole (Figure 1) vary between:

  • δ18O –3.4 and –6.3 ‰
  • δ13C –2.0 and +3.2 ‰

This suggests a δ18O seawater value of around –1.8‰ VSMOW and points to the occurrence of ice-free conditions in the late Serpukhovian.

Observed and consistent increases occur in the δ13C signature of organic carbon in the Throckley Borehole (Serpukhovian to Bashkirian; around –24 to –22 ‰) and the Rowlands Gill Borehole (Serpukhovian; around –24 to –23 ‰). These suggest increasing atmospheric δ13C ratios with time, likely prompted by increased photosynthesis. The trend implies a fall in atmospheric pCO2 and is consistent with the large-scale burial of organic material, probably in increasing lycophyte-dominated coal forest.

The combined evidence suggests that large-scale glaciation was not initiated until early Bashkirian times, when falling pCO2 could have been responsible for widespread cooling and glaciation.

The scenario of coalescing upland ice caps through the Serpukhovian in line with decreasing pCO2 appears to be similar to the process that initiated later Antarctic glaciation during the Cenozoic.

Further information

Stephenson, M H, Angiolini, L, Cozar, P, Jadoul, F, Leng, M J, Millward, D and Chenery, S. 2010. Northern England Serpukhovian (early Namurian) farfield responses to southern hemisphere glaciation. Journal of the Geological Society, 167 1171—1184.

Stephenson, M H, Millward, D, Leng, M J and Vane, C H. 2008. Palaeoecological and possible evolutionary effects of early Namurian glacioeustatic cyclicity, Journal of the Geological Society, 165, 993—1005.

Contact

Please contact Prof Mike Stephenson for further information