We appreciate the comments by Shane et al. on our paper (Lee et al., 2004). Shane et al. raise several interesting points about the main conclusion of our paper, which is the possibility of a link between the oldest Toba tuff eruption and the Pleistocene climate, although their main criticisms were focused on whether tephra layer D or E from Ocean Drilling Program (ODP) core 758 is the better representative of the oldest Toba tuff. The following is our rebuttal to each of the points raised by Shane et al.
In addition to the data presented in our paper, some handpicked pure glass shards from both layer D and layer E of ODP Site 758, collected by Dr. Dehn, were analyzed recently for their chemical and isotopic compositions. In general, the glass shards from layers D and E are distinct in their Na2O/K2O ratio and K2O content, with lower Na2O/K2O (<0.8) and higher K2O (>4.5%) for layer D, as opposed to higher Na2O/K2O (>0.9) and lower K2O (<4.5%) in layer E. Consequently, the characteristics of lower Na2O/K2O and higher K2O are entirely consistent with those of the oldest Toba tuff (Chesner, 1988, 1998). Moreover, Sr isotopic composition is very different between the glass shards from layer D and layer E. The 87Sr/86Sr value of the glass shard in layer D is 0.71306 ± 0.00003, which is quite comparable to those of the oldest Toba tuff from Sumatra (Chesner, 1988). In contrast, the 87Sr/86Sr ratio of the glass shard in layer E is 0.70559 ± 0.00002, significantly lower than that of the oldest Toba tuff in Sumatra (Chesner, 1988). This is further illustrated in Figure 1, where K2O content is plotted against 87Sr/86Sr for each sample, and it is readily apparent that glass shards from layer D of ODP Site 758 are well correlated, while layer E bears no resemblance to the oldest Toba tuff in Sumatra.
We have also analyzed the tephra glass shards extracted from South China Sea core MD972142 at the depth of 39.04 m for their chemical and Sr isotopic compositions, and they are also plotted in Figure 1. This tephra glass from the South China Sea core MD972142 is characterized by Na2O/K2O ~0.59 and 87Sr/86Sr = 0.71573 ± 0.00002 (Fig. 1), which is again entirely comparable to that observed in oldest Toba tuff from Sumatra and layer D in ODP Site 758.
As stated previously, layer D of ODP Site 758 is the better representative of the oldest Toba tuff. Nevertheless, the tephra layer D of ODP Site 758 along with the similar tephra layers from cores 17957 and MD972142 all are sandwiched between the Brunhes-Matuyama geomagnetic boundary and the Australasian microtektite layer, (Fig. 2 of Lee et al., 2004) which have been dated at 778.0 ± 1.7 ka (Tauxe et al., 1996) and 803 ± 3 ka (Hou et al., 2000), respectively. Based on this, our previous conclusion that the age of the oldest Toba tuff should be no older than 800 ka (Lee et al., 2004) remains valid. However, this age is in conflict with that reported by Diehl et al. (1987), which was based on a biotite cooling age dated at 840 ± 30 ka. Since all the ages are from literature data, we have no explanation for the discrepancy at this stage. Apparently, additional dating for the oldest Toba tuff in Sumatra and also the tephras from these drill cores is necessary in order to resolve the discrepancy of these studies and to provide better constraints for the timing of oldest Toba tuff eruption.
The sedimentation rate in the Central Indian Ocean basin is generally very slow, ~2–4 m/m.y. (Banakar et al., 1991). Three out of the eight cores inferred in the Banakar et al. study show evidence of severe bioturbation and current erosion (Banakar et al., 1991). In addition, microtektites have been found in the surface sediments (Prasad, 1994), and thus one should not be surprised if glass shards associated with the oldest Toba tuff are found in these drill cores. Similarly, in the case of Peninsula India, only two out of the seven sample sites have good glass fission-track ages (Westgate et al., 1998). As a result, the rest of the ash sites remain problematic due to the lack of good and systematic geochemical, isotopic, and age data. Although more detailed dating, chemical, and isotopic work for these ash deposits is needed to fully understand the nature and influences of Toba eruptions on the global climate, all the points raised by Shane et al. are speculative, and the conclusions of our previous paper remain valid.