Reply to Comment by Roberta M. Hotinski, Lee R. Kump, and Karen L. Bice on ‘‘Could the Late Permian deep ocean have been anoxic?’’

[1] Two recent studies [Hotinski et al., 2001; Zhang et al., 2001] have modeled and discussed Late Permian ocean circulation and oxygenation. These studies have reached significantly different conclusions. Zhang et al. [2001] find that a ‘‘thermal mode’’ ocean circulation driven by cooling in polar latitudes is unlikely to support deep-sea anoxia, but a ‘‘haline mode’’ ocean circulation, a shallow overturning cell driven by enhanced evaporation from the subtropics, perhaps could lead to global-scale deep-sea anoxia. Hotinski et al. [2001], using a different model, find that a ‘‘thermal mode’’ ocean circulation with a high equator-to-pole gradient gives well-oxygenated Permian deep water, while a ‘‘thermal mode’’ ocean circulation with a very low equator-to-pole gradient leads to global deep-sea anoxia. This discrepancy prompted a comment by Hotinski et al. [2002]. [2] In response, we would like to begin by emphasizing the common underlying feature of the two studies. In both models, deep ocean oxygen concentrations are significantly reduced if the overturning circulation becomes weaker with warmer, less oxygenated waters ventilating the deep ocean. In some circumstances this can lead to a dominance of respiration of organic matter and a very significant depletion of deep ocean oxygen. However, as noted above, the two studies differ greatly in the nature of the ocean circulation in which such a scenario can be achieved. [3] Hotinski et al. [2002] argue that the difference between the two model results are ‘‘in large part the result of dissimilar definitions of ‘weak’ or ‘reduced’ meridional temperature gradients.’’ We agree that the two studies apply different pole-equator temperature gradients at the surface. The high-latitudes surface temperature used to force the model in our study [Zhang et al., 2001, Figure 5b] is much warmer than that of modern climate and is based on an atmospheric circulation model of the Late Permian [Kutzbach and Gallimore, 1989], corresponding to a warm climate with enhanced greenhouse effect (5 CO2) and reduced solar luminosity (1% decrease). The high-latitudes surface temperature is more than 10 C (15 C) warmer than modern climate at 70 N (70 S). The warmer high-latitudes surface temperature is also similar to that of another modeling study of the Late Permian climate [Kutzbach and Ziegler, 1993], and is consistent with the Late Permian paleobotanical data interpreted by Rees et al. [1999]. The tropical surface temperature is about 3 C warmer than present, resulting in pole-equator temperature gradient that is weaker than that of modern climate. On the other hand, our modeled Late Permian polar sea surface temperature (SST) is cool relative to of the ‘‘low-gradient’’ scenario of Hotinski et al. [2001] forced by the estimated PaleoceneEocene thermal maximum (PETM) SST [Bice and Marotzke, 2001]. [4] If we use much higher polar surface temperature to force the model to give the estimated PETM polar SST as in the work by Hotinski et al. [2001], the thermal mode ocean circulation would be weakened. However, our recent theoretical study [Zhang et al., 2002] showed that when the pole-equator surface temperature gradient is reduced to a certain threshold, the weakened ‘‘thermal mode’’ ocean circulation would switch to the ‘‘haline mode’’ circulation, and the critical freshwater flux required for the ocean to switch to the haline mode circulation is much smaller with the reduced pole-equator surface temperature gradient. In that case, the deep ocean oxygen distribution would be similar to what we found with the haline mode circulation. Now the issue is when the pole-equator surface temperature gradient is larger than the threshold of mode switching, whether the weakened thermal mode circulation (with a maximum of 25 Sv as shown by Hotinski et al. [2001]) originating from warm polar surface water can drive global deep-sea anoxia. We note that the weakened thermal mode PALEOCEANOGRAPHY, VOL. 18, NO. 4, 1095, doi:10.1029/2002PA000851, 2003