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Reply to comment on: “The organic carbon isotopic and paleontological record across the Triassic–Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA” by Ward et al. (2007)
Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 205–206
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Reply to comment on: “The organic carbon isotopic and paleontological record across the Triassic–Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA” by Ward et al. (2007) Peter Ward a,⁎, Christopher McRoberts b, Kenneth Williford c a b c
Department of Biology, The University of Washington, Seattle WA 98195, USA Department of Geology, SUNY at Cortland, Cortland, New York, 13045, USA Department of Earth and Space Sciences, The University of Washington, Seattle WA 98195, USA
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 7 January 2008
We welcome the comment by Guex et al. (2009-this issue) in that it is clear that our publication (Ward et al., 2007) stimulated them to conduct additional research that augments results reported in their original papers (Guex et al., 2003, 2004). This example shows the value of separate teams investigating important sections. Unfortunately, we believe that their new interpretation of the most important aspect of this research, the position of the lowest carbon isotope anomaly with respect to the TJ boundary, remains in error. In spite of this, however, the decades of work on these sections by J. Guex and his various colleagues including his co-authors in the comment has resulted in a great increase in our understanding of many aspects of the Triassic/ Jurassic transition that will remain no matter the outcome of this current dispute about the carbon isotope record at Muller Canyon. The first carbon isotope measurement from Muller Canyon by Guex et al. (2003) interpreted a single point low in their section as a carbon isotope anomaly. In fact, the stratigraphic position of this reported anomaly was so different from other TJ boundary sections that it partly stimulated our own work to see if we could confirm this finding. We are happy to see that this lowest anomaly, based on a single datum point, is in their comment no longer interpreted as a carbon isotope anomaly. In their new curve presented above they now use a different method (three point running average) to construct a curve that defines data they interpret as anomalous to a background mean value. Their new data and new interpretation of their old data thus results in a different isotope curve from that of their original publication (it should be noted that not all authors of their comment were authors of the original study). However, like their old curve, their new curve remains different from ours, and in their comment, Guex et al. attempt to show why. DOI of original articles: 10.1016/j.palaeo.2006.06.042 and 10.1016/j.palaeo.2008.01.010. ⁎ Corresponding author. E-mail address: [email protected] (P. Ward). 0031-0182/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.palaeo.2008.01.002
Guex et al. (2009-this issue) propose that the differences in the positions of the lowest carbon isotope anomaly resulted from our not taking into account a doubling of several meters of section caused by a fault low in the section which caused us to add additional thickness to the Muller Canyon member. One of us (McRoberts) has recently been to the section and agrees that a small fault does exist in part of the section where paleontological samples were collected in 2005. The original geochemcial sampling and stratigraphic log in Ward et al. (2007) were made by P. Ward and D. Kring in 2003 from a 1–2 m deep trench (later covered). Our geochemical samples did not come from the yellow path shown in the figure in their response, but from a trench dug well to the right on their photo, in a region where their fault is not exposed. The trench referred to in the comment above was dug to obtain paleontological rather than geochemical samples. Guex et al. (2009-this issue) also point out what they view as three paleontological problems in our paper. First, they report that we identified one group of fossils in the lowest part of the Muller Canyon member as belemnites. This is a trivial point with no bearing on the chronostratigraphy argument or the matter at hand here: they are coleoids of some sort with no present biostratigraphic value. Yet while their identification is of no importance for defining the TJ boundary or any other biostratigraphic marker, the bed that they come from is important to the lithostratigraphy and correlation of the sections along strike. It is distinctive marker bed occurring below the fault, and is an excellent marker for the base of the Muller Canyon member. Secondly, Guex et al. (2009-this issue) suggest that our determination of a specimen as Psiloceras and figured in or paper was in error. In fact the very poor preservation of the specimens from this level preclude assignment to a species, and trying to identify this specimen from our photograph is impossible: we will happily send it to them for identification and acknowledge their expertise in the classification of ammonites from this locality. However we suggest that the true identification of this specimen is irrelevant, but related to the third of
206
P. Ward et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 205–206
the paleontological disagreements noted by Guex et al. (2009-this issue). Guex et al. (2009-this issue) contend that the bivalve Aegerchlamys has no biostratigraphic value for marking the base of the Jurassic. We disagree. We followed McRoberts (2004) in using the pectinacean bivalve Agerchlamys as a proxy for the base of a paleontologicallydefined Jurassic in this section. We have not advocated for using Agerchlamys as the defining criterion for a GSSP for the base Hettangian (see also McRoberts et al., 2007). Contrary to the assertion in Lucas et al. (2007a) and Lucas and Tanner (2007), there are no published accounts of Agerchlamys in undisputed Triassic strata (see comments in McRoberts et al., 2007 and, for the New York Canyon section in particular, the discussion in Lucas et al., 2007b). The one Triassic species, Chlamys (Camptochlamys) inspecta Kiparisova (in Kiparisova et al., 1966) listed by Damborenea (1993) in her original definition of the Agerchlamys lacks radial ornament on its auricles and is considered to belong to a different genus. Because many of the lowest Jurassic ammonites have little ornamentation or other characters for taxonomic identification, excellent 3-D preservation with preserved suture lines are essential. Unfortunately, most ammonites from the Muller Canyon member are flattened to some degree (so that no original cross section shape is obtainable), and sutures are only rarely preserved in ammonites from the Muller Canyon Member. The most important point is that even with the identified fault, the two curves differ. We still interpret our negative anomaly to occur just beneath the first occurrence of Aegerchlamys that we collected in 2005, and thus our anomaly is essentially co-incident (in fact, just below) with the first marker for the Jurassic. This differs markedly from the Guex et al. findings but is consistent with the results from Kennecott Point, Queen Charlotte Islands, as well as some other sections. Because many of the authors on the Guex et al. (2009-this issue) reply are also involved in descriptions of the Muller Canyon section, and have recently used the original Guex et al. curve rather than ours as the primary data about isotopes from this section (Lucas et al., 2007a,b), we are discouraged that our results with data judged to have less scatter even by the Guex et al. team have been ignored. That their negative anomaly remains well below ours in this section will
perpetuate a serious error if it is accepted as the primary carbon isotope data for this section. To conclude this dispute we advocate a “blind sampling” program by an independent researcher. Because Muller Canyon is a viable candidate for the GSSP stratotype, this extra measure of research is warranted. We welcome a repeat of our sampling section measurement as well as laboratory analyses. References Damborenea, S.E., 1993. Early Jurassic South American pectinaceans and circum-Pacific palaeobiogeography. Palaeogeography, Palaeoclimatology, Palaeoecology 100. Guex, J., Bartolini, A., Atudorei, V., Taylor, D., 2003. Two negative δ13Corg excursions near the Triassic–Jurassic boundary in the New York Canyon area (Gabbs Valley Range, Nevada). Bulletin des Géologie, Minéralogie, Géophysique et du Musée géologique de l'Université Lausanne, vol. 360, pp. 1–4. Guex, J., Bartolini, A., Atudorei, V., Taylor, D.G., 2004. High-resolution ammonite and carbon isotope stratigraphy across the Triassic–Jurassic boundary at New York Canyon (Nevada). Earth and Planetary Science Letters 225, 29–41. Guex, J., Bartolini, A., Taylor, D., Atudorei, V., Thelin, P., Bruchez, S., Tanner, L., Lucas, S., 2009. Comment on: “The organic carbon isotopic and paleontological record across the Triassic-Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA” by Ward et al. 2007. Palaeogeogr. Palaeoclimatol. Palaeoecol. 273, 205–206 (this issue). Kiparisova, L.D., Bychkov, Y.M., Polubotko, I.V., 1966. Upper Triassic bivalve molluscs from the northeast USSR. Vsesoyuznyy nauchno-issledovatel'skii instituta (VSEGEI), Magadan. 312 pp. Lucas, S.G., Tanner, L.H., 2007. The nonmarine Triassic–Jurassic boundary in the Newark Supergroup of eastern NorthAmerica. Earth Science Reviews 84, 1–20. Lucas, S.G., Taylor, D.G., Guex, J., Tanner, L.H., Krainer, K., 2007a. The proposed Global Stratotype Section and Point for the base of the Jurassic System in the New York Canyon Area, Nevada, USA. In: Lucas, S., Spielmann (Eds.), Triassic of the American West, New Mexico Museum of Natural History & Science Bulletin, no. 40, pp. 139–161. Lucas, S.G., Taylor, D.G., Guex, J., Tanner, L.H., Krainer, K., 2007b. Updated proposal for Global Stratotype Section and Point for the base of the Jurassic System in the New York Canyon Area, Nevada, USA., international subcommission on Jurassic stratigraphy. Newsletter 34 (1), 34–42. McRoberts, C.A., 2004. Marine bivalves and the end-Triassic mass extinction: Faunal turnover, isotope anomalies and implications for the position of the Triassic/ Jurassic boundary. Abstracts of the 32nd International Geological Congress, p. 1139. McRoberts, C.A., Ward, P.D., Hesselbo, S., 2007. A proposal for the base Hettangian Stage (=base Jurassic System) GSSP at New York Canyon (Nevada, USA) using carbon isotopes. International subcommission on Jurassic stratigraphy. Newsletter 34 (1), 43–49. Ward, P.D., Garrison, G.H., Williford, K.H., Kring, D.A., Goodwin, D., Beattie, M.J., McRoberts, C.A., 2007. The organic carbon isotopic and paleontological record across the Triassic–Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA. Palaeogeogr. Palaeoclimatol. Palaeoecol. 244 (1–4), 281–289.
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Reply to comment on: “The organic carbon isotopic and paleontological record across the Triassic–Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA” by Ward et al. (2007) Peter Ward a,⁎, Christopher McRoberts b, Kenneth Williford c a b c
Department of Biology, The University of Washington, Seattle WA 98195, USA Department of Geology, SUNY at Cortland, Cortland, New York, 13045, USA Department of Earth and Space Sciences, The University of Washington, Seattle WA 98195, USA
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 7 January 2008
We welcome the comment by Guex et al. (2009-this issue) in that it is clear that our publication (Ward et al., 2007) stimulated them to conduct additional research that augments results reported in their original papers (Guex et al., 2003, 2004). This example shows the value of separate teams investigating important sections. Unfortunately, we believe that their new interpretation of the most important aspect of this research, the position of the lowest carbon isotope anomaly with respect to the TJ boundary, remains in error. In spite of this, however, the decades of work on these sections by J. Guex and his various colleagues including his co-authors in the comment has resulted in a great increase in our understanding of many aspects of the Triassic/ Jurassic transition that will remain no matter the outcome of this current dispute about the carbon isotope record at Muller Canyon. The first carbon isotope measurement from Muller Canyon by Guex et al. (2003) interpreted a single point low in their section as a carbon isotope anomaly. In fact, the stratigraphic position of this reported anomaly was so different from other TJ boundary sections that it partly stimulated our own work to see if we could confirm this finding. We are happy to see that this lowest anomaly, based on a single datum point, is in their comment no longer interpreted as a carbon isotope anomaly. In their new curve presented above they now use a different method (three point running average) to construct a curve that defines data they interpret as anomalous to a background mean value. Their new data and new interpretation of their old data thus results in a different isotope curve from that of their original publication (it should be noted that not all authors of their comment were authors of the original study). However, like their old curve, their new curve remains different from ours, and in their comment, Guex et al. attempt to show why. DOI of original articles: 10.1016/j.palaeo.2006.06.042 and 10.1016/j.palaeo.2008.01.010. ⁎ Corresponding author. E-mail address: [email protected] (P. Ward). 0031-0182/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.palaeo.2008.01.002
Guex et al. (2009-this issue) propose that the differences in the positions of the lowest carbon isotope anomaly resulted from our not taking into account a doubling of several meters of section caused by a fault low in the section which caused us to add additional thickness to the Muller Canyon member. One of us (McRoberts) has recently been to the section and agrees that a small fault does exist in part of the section where paleontological samples were collected in 2005. The original geochemcial sampling and stratigraphic log in Ward et al. (2007) were made by P. Ward and D. Kring in 2003 from a 1–2 m deep trench (later covered). Our geochemical samples did not come from the yellow path shown in the figure in their response, but from a trench dug well to the right on their photo, in a region where their fault is not exposed. The trench referred to in the comment above was dug to obtain paleontological rather than geochemical samples. Guex et al. (2009-this issue) also point out what they view as three paleontological problems in our paper. First, they report that we identified one group of fossils in the lowest part of the Muller Canyon member as belemnites. This is a trivial point with no bearing on the chronostratigraphy argument or the matter at hand here: they are coleoids of some sort with no present biostratigraphic value. Yet while their identification is of no importance for defining the TJ boundary or any other biostratigraphic marker, the bed that they come from is important to the lithostratigraphy and correlation of the sections along strike. It is distinctive marker bed occurring below the fault, and is an excellent marker for the base of the Muller Canyon member. Secondly, Guex et al. (2009-this issue) suggest that our determination of a specimen as Psiloceras and figured in or paper was in error. In fact the very poor preservation of the specimens from this level preclude assignment to a species, and trying to identify this specimen from our photograph is impossible: we will happily send it to them for identification and acknowledge their expertise in the classification of ammonites from this locality. However we suggest that the true identification of this specimen is irrelevant, but related to the third of
206
P. Ward et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 205–206
the paleontological disagreements noted by Guex et al. (2009-this issue). Guex et al. (2009-this issue) contend that the bivalve Aegerchlamys has no biostratigraphic value for marking the base of the Jurassic. We disagree. We followed McRoberts (2004) in using the pectinacean bivalve Agerchlamys as a proxy for the base of a paleontologicallydefined Jurassic in this section. We have not advocated for using Agerchlamys as the defining criterion for a GSSP for the base Hettangian (see also McRoberts et al., 2007). Contrary to the assertion in Lucas et al. (2007a) and Lucas and Tanner (2007), there are no published accounts of Agerchlamys in undisputed Triassic strata (see comments in McRoberts et al., 2007 and, for the New York Canyon section in particular, the discussion in Lucas et al., 2007b). The one Triassic species, Chlamys (Camptochlamys) inspecta Kiparisova (in Kiparisova et al., 1966) listed by Damborenea (1993) in her original definition of the Agerchlamys lacks radial ornament on its auricles and is considered to belong to a different genus. Because many of the lowest Jurassic ammonites have little ornamentation or other characters for taxonomic identification, excellent 3-D preservation with preserved suture lines are essential. Unfortunately, most ammonites from the Muller Canyon member are flattened to some degree (so that no original cross section shape is obtainable), and sutures are only rarely preserved in ammonites from the Muller Canyon Member. The most important point is that even with the identified fault, the two curves differ. We still interpret our negative anomaly to occur just beneath the first occurrence of Aegerchlamys that we collected in 2005, and thus our anomaly is essentially co-incident (in fact, just below) with the first marker for the Jurassic. This differs markedly from the Guex et al. findings but is consistent with the results from Kennecott Point, Queen Charlotte Islands, as well as some other sections. Because many of the authors on the Guex et al. (2009-this issue) reply are also involved in descriptions of the Muller Canyon section, and have recently used the original Guex et al. curve rather than ours as the primary data about isotopes from this section (Lucas et al., 2007a,b), we are discouraged that our results with data judged to have less scatter even by the Guex et al. team have been ignored. That their negative anomaly remains well below ours in this section will
perpetuate a serious error if it is accepted as the primary carbon isotope data for this section. To conclude this dispute we advocate a “blind sampling” program by an independent researcher. Because Muller Canyon is a viable candidate for the GSSP stratotype, this extra measure of research is warranted. We welcome a repeat of our sampling section measurement as well as laboratory analyses. References Damborenea, S.E., 1993. Early Jurassic South American pectinaceans and circum-Pacific palaeobiogeography. Palaeogeography, Palaeoclimatology, Palaeoecology 100. Guex, J., Bartolini, A., Atudorei, V., Taylor, D., 2003. Two negative δ13Corg excursions near the Triassic–Jurassic boundary in the New York Canyon area (Gabbs Valley Range, Nevada). Bulletin des Géologie, Minéralogie, Géophysique et du Musée géologique de l'Université Lausanne, vol. 360, pp. 1–4. Guex, J., Bartolini, A., Atudorei, V., Taylor, D.G., 2004. High-resolution ammonite and carbon isotope stratigraphy across the Triassic–Jurassic boundary at New York Canyon (Nevada). Earth and Planetary Science Letters 225, 29–41. Guex, J., Bartolini, A., Taylor, D., Atudorei, V., Thelin, P., Bruchez, S., Tanner, L., Lucas, S., 2009. Comment on: “The organic carbon isotopic and paleontological record across the Triassic-Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA” by Ward et al. 2007. Palaeogeogr. Palaeoclimatol. Palaeoecol. 273, 205–206 (this issue). Kiparisova, L.D., Bychkov, Y.M., Polubotko, I.V., 1966. Upper Triassic bivalve molluscs from the northeast USSR. Vsesoyuznyy nauchno-issledovatel'skii instituta (VSEGEI), Magadan. 312 pp. Lucas, S.G., Tanner, L.H., 2007. The nonmarine Triassic–Jurassic boundary in the Newark Supergroup of eastern NorthAmerica. Earth Science Reviews 84, 1–20. Lucas, S.G., Taylor, D.G., Guex, J., Tanner, L.H., Krainer, K., 2007a. The proposed Global Stratotype Section and Point for the base of the Jurassic System in the New York Canyon Area, Nevada, USA. In: Lucas, S., Spielmann (Eds.), Triassic of the American West, New Mexico Museum of Natural History & Science Bulletin, no. 40, pp. 139–161. Lucas, S.G., Taylor, D.G., Guex, J., Tanner, L.H., Krainer, K., 2007b. Updated proposal for Global Stratotype Section and Point for the base of the Jurassic System in the New York Canyon Area, Nevada, USA., international subcommission on Jurassic stratigraphy. Newsletter 34 (1), 34–42. McRoberts, C.A., 2004. Marine bivalves and the end-Triassic mass extinction: Faunal turnover, isotope anomalies and implications for the position of the Triassic/ Jurassic boundary. Abstracts of the 32nd International Geological Congress, p. 1139. McRoberts, C.A., Ward, P.D., Hesselbo, S., 2007. A proposal for the base Hettangian Stage (=base Jurassic System) GSSP at New York Canyon (Nevada, USA) using carbon isotopes. International subcommission on Jurassic stratigraphy. Newsletter 34 (1), 43–49. Ward, P.D., Garrison, G.H., Williford, K.H., Kring, D.A., Goodwin, D., Beattie, M.J., McRoberts, C.A., 2007. The organic carbon isotopic and paleontological record across the Triassic–Jurassic boundary at the candidate GSSP section at Ferguson Hill, Muller Canyon, Nevada, USA. Palaeogeogr. Palaeoclimatol. Palaeoecol. 244 (1–4), 281–289.