Pufahl et al. (2010) question the interpretation of data in Poulton et al. (2004), showing a transition from ferruginous to euxinic conditions on the continental shelf of a major ocean basin at ca. 1840 Ma. In contrast with Poulton et al. (2004) and also Johnston et al. (2006), Pufahl et al. conclude that the study area, the Animikie Basin (Canada and the United States), was at least partially isolated from the world ocean, allowing development of local anomalous water chemistry not reflective of the open ocean. They argue, from new S isotope data, that restricted connection to the open ocean allowed dilution of basin waters in shore-proximal settings. This led, in their view, to lower pyrite δ34S in sediments deposited in the southern portion of the basin (the Michigamme Formation) compared to time-correlative units to the north, which were presented by Poulton et al. (2004) and Johnston et al. (2006). The interpretation of Pufahl et al. requires a landmass to the south or southwest, in the area of the Penokean Orogen. This landmass is needed to supply fresh water to the area of Michigamme Formation sedimentation and to provide a barrier to the open ocean further to the south.
There are four major difficulties with this view:
1) Paleocurrent studies of the delta-submarine fan system comprising the upper portion of the succession show flow was from the north, not south (Morey, 1973). This implies that the sediment was being supplied from the Trans-Hudson Orogen, not the Penokean (Maric and Fralick, 2005).
2) Proximal to distal lithologic changes in rocks of the Rove and Virginia Formations, recorded in a transect across the Animikie shelf, clearly show a north-to-south fining trend only compatible with sediment feed from the north (Maric and Fralick, 2005; Poulton et al., 2010).
3) The ∼100 m of carbonaceous shales and siltstones overlying the Sudbury ejecta horizon at the base of the Rove and Virginia Formations have been interpreted as a sediment-starved, condensed interval (Johnston et al., 2006,), but Pufahl et al. (2010, their Figure 3) interpret correlative rocks in the Michigamme Formation as peritidal deposits. U-Pb zircon geochronology on tuffs indicate sedimentation of this interval began at ca. 1832 Ma (Addison et al., 2005), while the youngest ages are 1780 Ma for detrital zircons in turbiditic sandstones conformably overlying this interval (Heaman et al., 2005; Wirth et al., 2006). The time interval for deposition, a minimum of ∼50 m.y., is consistent with a thick, sediment-starved condensed interval, not peritidal deposits. While it is likely that the lowest few meters are peritidal, as appears to be the case in the Rove and Virginia Formations, the overlying, finer-grained units must represent a sub-tidal condensed system to accommodate the large time interval represented by these rocks.
4) Penokean deformation and metamorphism severely affect
sthe Michigamme Formation, indicating that uplift occurred after deposition. Since the orogeny is at least 50 m.y. younger than the beginning of sedimentation of the Michigamme Formation, orogeny-related uplift could not have generated a land area to supply fresh water to the Michigamme and to isolate the flooded edge of the continent from the deep ocean.
Pufahl et al. are commended for emphasizing the importance of developing data sets showing three-dimensional regional variability. However, the additional lateral data provided by core DL-4B (Pufahl et al., 2010) while useful, highlight the limitations of using S isotopes alone as an indicator of spatial variability in oceanic redox conditions. Sulfur isotope compositions of marine sediments are potentially controlled by a wide variety of processes. Therefore, Poulton et al. (2004) used S isotope data to support evidence for euxinic conditions on the shallow shelf, while independent Fe speciation data was used to drive interpretations of water chemistry. Rather than using S isotope compositions in isolation, multi-proxy analyses of samples from a series of cores extending from the most shore proximal position in the north to the open, outer shelf in the south would provide the most valuable information. This has been done by Poulton et al. (2010). The additional data show that the S isotope composition of pyrite becomes heavier as sulfate is removed by bacterial sulfate reduction during transit across the continental shelf and through redox conditions ranging from Fe-rich in the deeper waters to euxinic in shallower waters. These trends in S isotopic compositions are completely consistent with an open-ocean setting and are not obviously supported by the fresh-water dilution model of Pufahl et al. The data of Pufahl et al., however, fit well into the interpretations presented in Poulton et al. (2010). Thus, we are further convinced that our general conclusions put forward in 2004 and elaborated on in 2010 are correct.