The onset of OAE 2 is identified by an abrupt 2–3‰ VPDB δ13Corg positive shift from values of ∼−27‰ in the upper Hartland Shale, 4.3 m below the CTB (Fig. 2; Supplementary Material, Table 1a; Sageman et al., 2006). The positive excursion is characteristic of the isotopic response during OAE 2 and, although many localities record increased organic carbon deposition at this level (e.g., Tsikos et al., 2004), sites within the WIS do not. Here the onset is characterized by organic-poor interbedded limestones and shales that are generally bioturbated. Shale interbeds in the upper half of the OAE 2 interval, however, do become enriched in TOC in the WIS. The end of OAE 2 is expressed by a gradual fall in δ13Corg back to ∼−27‰ (Sageman et al., 2006).
VPDB δ13Corg
positive shift
2–3‰
‰
upper Hartland Shale
VPDB δ13Corg
∼−27‰
‰
upper Hartland Shale
below the CTB
4.3 m
m
δ13Corg
gradual fall
∼−27‰
‰