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the Creative Commons Attribution 4.0 License.
| 01 Dec 2021
Palaeobiological evidence for Southern Hemisphere Younger Dryas and volcanogenic cold periods
Abstract. Current consensus places a Southern Hemisphere post-glacial cooling episode earlier than the Younger Dryas in the Northern Hemisphere. New Zealand sequences of glacial moraines and speleothem isotopic data are generally interpreted as supporting the absence of a Southern Hemisphere Younger Dryas. Radiocarbon age series of habitat specialist moa (Aves: Dinornithiformes) show, however, that a sudden return to glacial climate in central New Zealand contemporary with the Younger Dryas. The cooling followed significant warming, not cooling, during the period of the Antarctic Cold Reversal. In addition, the moa sequence chronology also shows that the Oruanui (New Zealand) and Mt Takahe (Antarctica) volcanic eruptions were contemporary with abrupt cooling events in New Zealand. The independent high spatial and temporal resolution climate chronology reported here is contrary to an inter-hemispheric post-glacial climate see-saw model.
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RC1: 'Comment on cp-2021-154', Matt McGlone, 09 Jan 2022
General
This ms compiles and analyses previously published data on the occurrence of moa in cave deposits from the northern half of the South Island of New Zealand. On the basis of the assumed high habitat specificity of the moa species, and with reference to local d O18 speleothem records, a sequence of vegetation and climate change is deduced for the Late Pleistocene-early Holocene. In contrast to the developing consensus derived from New Zealand marine cores, glacial advances and pollen sequences from the South Island, it is claimed on basis of the moa dates that the Antarctic Cold Reversal (ACR: c. 14.5 – 12.8 kyr) was represented by a local warming and that the Younger Dryas (YD: 12.8-11.5 kyr) by a return to glacial conditions. While the author is to be congratulated on bringing together a comprehensive fossil moa data set to address the question of Late Pleistocene climate change, his failure to discuss or even cite the relatively large amount of detailed paleoclimatic data from this region which would lead to diametrically opposed conclusions makes his case untenable.
Specific comments
1Speleothem interpretation
The interpretation of the speleothem records in this paper assumes that they are entirely driven by local temperature change. This is a flawed assumption, as speleothem records are influenced by many factors ranging from distant oceanic moisture sources, rainout through to those affecting soil infiltration and precipitation in the cave system. “The immediate forcing factors for the observed palaeoclimatic variations are therefore most likely related mainly to changes in ocean source waters of precipitation, transmitted by westerly winds.” (Williams et al. 2010). As the late Pleistocene was a time of major changes in wind and oceanic water masses, it is highly likely that these rather than local temperature drove the changes. The current ms suggests that there was a warming between 14 and 13 ka followed by a cooling between 13 and 12 ka (Fig. 12). The interpretation given by those who generated the speleothem data does not entirely support this interpretation: “Late-glacial warming commenced between 18.2 and 17.8 ka and accelerated after 16.7 ka, culminating in a positive excursion between 14.70 and 13.53 ka. This was followed by a significant negative excursion between 13.53 and 11.14 ka of up to 0.55‰ depth that overlapped the Antarctic Cold Reversal (ACR) and spanned the Younger Dryas (YD). Positive δ18O excursions at 11.14 ka and 6.91–6.47 ka represent the warmest parts of the Holocene.” (Williams et al. 2005). Moreover, a careful examination of the curves in Williams et al. (2005) show that it was a period of warming, as those authors state: “We agree with the conclusion of Turney et al. that the YD chronozone in New Zealand was a period of resumed warming, because d18Oc values show an upwards trend after ~12.7 ka.”
2 Quality of the moa fossil record versus the pollen and macrofossil record
Some extraordinary claims are made in the ms on the value of the moa record versus the other palaeoclimatic indicators from the South Island and the surrounding ocean (lines 385-391):
“Unlike indirect proxies such as oxygen isotope ratios and pollen samples from a few sites, moa were on-ground real-time witnesses to the vegetation (and hence climate) at the time and place they lived and died. The congruence of the high spatial and temporal resolution record of vegetation based on 14 C ages of individual moa, each characteristic of a particular vegetation (Fig. 12) with the local speleothem δ18 O record suggest that both proxies accurately reflect the timing of changes in the New Zealand glacial and post-glacial climate. However, the postglacial return to cold climate in both records is later than the ACR. Instead, the moa sequence accords with the δ18 O record in showing that interval to be one of warming climate at these southern latitudes.” I make the following comments on these claims:
- The moa record is not particularly high resolution given that the dated remains are scattered through numerous cave systems.
- The argument that moa are a direct indicator of vegetation composition while pollen and macrofossils derived from that vegetation are indirect, is without merit. Moa like most large vertebrates would have used a variety of vegetation types and, rather than a documented analysis of the occurrence of moa in relation to vegetation, we have impressionistic claims. In contrast there is a long history in New Zealand of quantitative assessments of pollen rain versus vegetation type. Marine isotope and alkenone proxies likewise have an impressive, well understood, quantitative and physical basis.
- Interpolation of ages from dated horizons (which is critiqued here as inferior to inferences made on the basis of individual fossil dates as per the current ms) is a well established procedure and, although it can involve some inaccuracies, the mass of data now accumulated in the New Zealand region from dated cores is such that we can high confidence that the ACR was not a period of strong warming, whereas the YD was. Problems now largely centre not on the dating, but on the interpretation of how the various proxies react to climate forcing. For instance, some proxies represent a seasonal signal rather than an annual one.
3 Lack of discussion of other relevant climate proxies
The only local climate proxies discussed in detail here are dO18 speleothem records and moa fossils. I here briefly discuss some of the most relevant that are missing from such a discussion.
The results from glacier advance investigations are unequivocal (Doughty et al. 2013): “The modelling results presented in this paper supports the idea that the temperature during the ACR was 2-3 oC cooler than today in New Zealand. This temperature change estimate overlaps, within error, with a temperature change estimate of 3-4 oC cooler than today, reported by Anderson and Mackintosh (2006) for the Franz Josef Glacier advance to the Waiho Loop during the Lateglacial. Our results are also consistent with published pollen studies (Newnham and Lowe, 2000; Turney et al., 2003; Vandergoes and Fitzsimons, 2003; Hajdas et al., 2006), and regional sea-surface temperature records (Carter and Cortese, 2009; Sikes et al., 2009) that suggest a temporary reversal of the deglaciation warming trend occurred in New Zealand during the Lateglacial”
Marine core palaeoclimatic analyses directly to the west of the northern South Island also support the concept of an ACR cool period and strong warming during the YD: “There are no short-term fluctuations late in the Pleistocene that could potentially be a YD cooling event, even allowing for generous errors in the age model. Consistent with warming in the SST series, the YD interval appears to be associated locally with a period of maximum forest growth (increasing abundance of podocarps and hardwoods, Fig. 1), not cooling.” (Barrows et al. 2007).
A detailed pollen and macrofossil study (Jara et al. 2015) near treeline in Northwest Nelson, close to the moa caves that provide most of evidence in the current ms, also does not support the interpretation of a warm ACR and a cold YD. Temperatures modelled on the basis of pollen from Adelaide Tarn are well below current during the ACR, whereas the YD chronozone sees a rapid rise in temperature to above present day levels. On the central-eastern flank of the Southern Alps, pollen and macrofossil evidence point to a cool ACR and strong warming during the YD (McGlone et al. 2004).
Further south to the east of the South Island, marine core evidence (Pahnke et al. 2006; Pahnke, 2003) indicates continuous warming from 18 ka, but with a steep acceleration during the YD chronozone. The subantarctic Campbell Island pollen record immediately to the south of the South Island shows cool conditions during the ACR followed by steep warming during the YD (McGlone et al. 2010, McGlone, et al. 2019).
Concluding remarks
Overturning a broad consensus (based on numerous New Zealand late Pleistocene-early Holocene investigations of glacial moraines, deep sea cores and pollen and macrofossil cores) that the ACR in this region of the southwest Pacific was either cool or warming at a slower rate than the subsequent rapid temperature increase during the YD, requires much more substantial evidence than that presented here. The speleothem evidence cannot be attributed directly to temperate alone, and the most parsimonious explanation for the mis-match of the dO18 results from this source with other proxies is changing oceanic sources and wind flow. Moreover, even if it could be demonstrated that the moa fossils are reliable indicators of changing vegetation, given the low temporal resolution of the evidence presented and the fact that these were large, highly mobile animals, this evidence does not undermine the solid, well replicated, well dated paleotemperature records derived from moraines, deep-sea cores and pollen and macrofossils from adjacent South Island sites. Some reconciliation would be needed if the re-interpretation suggested were to be accepted, but this current ms barely recognizes the existence of this data, let alone attempts to discuss it.
I therefore see no reason based on the results in this ms to abandon the concept that temperature trends during the late Pleistocene-early Holocene in the New Zealand region were roughly in antiphase with those in the mid to high latitudes of Northern Hemisphere.
Matt McGlone, Manaaki Whenua-Landcare Research, Lincoln, New Zealand
Barrows, T. T., et al. (2007). "Absence of cooling in New Zealand and the adjacent ocean during the Younger Dryas chronozone." Science 318 (5847): 86-89.
Doughty, A. M., et al. (2013). "Evaluation of lateglacial temperatures in the Southern Alps of New Zealand based on glacier modelling at Irishman Stream, Ben Ohau Range." Quaternary Science Reviews 74: 160-169.
Jara, I. A., et al. (2015). "Pollen-climate reconstruction from northern South Island, New Zealand ( 41 degrees S), reveals varying high- and low-latitude teleconnections over the last 16 000 years." Journal of Quaternary Science 30(8): 817-829.
McGlone, M. S., et al. (2004). "Late-glacial and Holocene vegetation and climatic history of the Cass basin, central south island, New Zealand." Quaternary Research 62(3): 267-279.
McGlone, M. S., et al. (2010). "Divergent trends in land and ocean temperature in the Southern Ocean over the past 18,000 years." Nature Geoscience 3(9): 622-626.
McGlone, M. S., et al. (2019). "Temperature, wind, cloud, and the postglacial tree line history of sub-Antarctic Campbell Island." Forests 10(11): 998.
Pahnke, K. & Sachs, J. P. 2006. Sea surface temperatures of southern midlatitudes 0–160 kyr BP. Paleoceanography 21, PA2003. 5.
Pahnke, K., Zahn, R., Elderfield, H. & Schulz, M. 2003. 340,000-year centennial-scale marine record of Southern Hemisphere climatic oscillation. Science 301, 948–952
Williams, P. W., et al. (2005). "Late Pleistocene to Holocene composite speleothem 18O and 13C chronologies from South Island, New Zealand—did a global Younger Dryas really exist?" Earth and Planetary Science Letters 230(3-4): 301-317.
Williams, P. W., et al. (2010). "Age frequency distribution and revised stable isotope curves for New Zealand speleothems: palaeoclimatic implications." International Journal of Speleology 39(2): 99-112.
Citation: https://doi.org/10.5194/cp-2021-154-RC1 - AC1: 'Reply on RC1', Richard Holdaway, 16 Feb 2022
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RC2: 'Comment on cp-2021-154', Ignacio Jara, 10 Feb 2022
Overview
The author presents a series of radiocarbon chronologies of fossil moa remains covering the Last Glacial Maximum (LGM) and the Last Termination (LT) from several fossil sites in the South Island of New Zealand. The climate interpretation of the 14C sequences assumes that moa species are in strict association with specific vegetation types; and uses New Zealand’s δ18O speleothem series as a master temperature record. The moa chronologies are then evaluated in the light of long-lasting debate about temperature changes during lateglacial times in New Zealand, coming up with a rather controversial claim of rapid warming during the Antarctic Cold Reversal (ACR; 14.6-13 kcal BP) and significant cooling aligned with the Younger Dryas chronozone (YD; 12.9-11.7 kcal BP).
Although the use of habitat-specialist moa species as a proxy for temperature change is definitely a novel approach, and even though the author have made a comprehensive revision of the published radiocarbon chronologies; the manuscript does not seem to provide the sufficient evidence and the necessary revision of the vast literature to sustain some of its main conclusions.
For instance, the author argues than moa fossil assemblages are a more direct climate proxy than pollen or isotopes records, yet this hard to justify considering that the moa’s habitat specificity could have certainly changed in the face of rapid landscape changes, and that the chronologies presented in the manuscript represent a discontinuous record of individual, ephemeral paleoecological events.
The results presented by the author challenge the present model of inter-hemispheric relationship between climate events during the LT, and therefore hold large implications for the Quaternary paleoclimate community. To sustain such a big claim, a convincing explanation about why significant warming (cooling) occurred in NZ during the ACR (YD) at the same time than several other mid-latitude terrestrial records were documenting significant cooling (warming) is essential. Unfortunately, it is missing in the text.
Specific comments
Lack of a hemispheric perspective
The results presented in the manuscript challenge the current consensus about the climate events that characterised LT in the Southern Hemisphere. In this regard, the author fails to place the NZ moa chronology within a continental paleoclimatic context. There is solid evidence, from a good number of high resolution, well-dated climate records, to sustain an extension of the Antarctic deglaciation pattern into the Southern Hemisphere mid-latitudes; however, only a very small number of studies are mentioned or discussed in the main text.
I suggest the inclusion and thorough revision of the some of the most recent literature concerning this topic, included but not limited to:
- Fletcher, M.S., Pedro, J., Hall, T., Mariani, M., Alexander, J.A., Beck, K., Blaauw, M., Hodgson, D.A., Heijnis, H., Gadd, P.S. and Lise-Pronovost, A., 2021. Northward shift of the southern westerlies during the Antarctic Cold Reversal. Quaternary Science Reviews, 271, p.107189.
- Mendelova, M., Hein, A.S., Rodes, A., Smedley, R.K. and Xu, S., 2020. Glacier expansion in central Patagonia during the Antarctic Cold Reversal followed by retreat and stabilisation during the Younger Dryas. Quaternary Science Reviews, 227, p.106047.
- Sagredo, E.A., Kaplan, M.R., Araya, P.S., Lowell, T.V., Aravena, J.C., Moreno, P.I., Kelly, M.A. and Schaefer, J.M., 2018. Trans-pacific glacial response to the Antarctic Cold Reversal in the southern mid-latitudes. Quaternary Science Reviews, 188, pp.160-166.
- Hinojosa, J.L., Moy, C.M., Vandergoes, M., Feakins, S.J. and Sessions, A.L., 2019. Hydrologic change in New Zealand during the last deglaciation linked to reorganization of the Southern Hemisphere westerly winds. Paleoceanography and Paleoclimatology, 34(12), pp.2158-2170.
- Moreno, P.I., Denton, G.H., Moreno, H., Lowell, T.V., Putnam, A.E. and Kaplan, M.R., 2015. Radiocarbon chronology of the last glacial maximum and its termination in northwestern Patagonia. Quaternary Science Reviews, 122, pp.233-249.
As the evidence presented in this manuscript is not discussed in the light of these (and several others) studies, the author omits an explanation for why the Moa chronologies suggest a warming-cooling pattern during the ACR-YD intervals, whilst terrestrial records from other southern mid-latitudes regions indicates the opposite climate pattern.
In addition, no Antarctic ice core data has been included in the figures or discussed in the main text. It is rather surprising that the manuscript places so much attention to the Greenland ice core data without mentioning or discussing the detailed Antarctic ice core isotope or gas timeseries. It seems that the author is over-stressing the data that agrees with its interpretation of the moa sequences (i.e., NZ speleothems, Greenland ice cores), to the detriment of a great number of detailed and well-dated records from NZ, the mid-latitudes, and Antarctica.
Radiocarbon sequence of moa as a proxy for hemispheric climate events
The author indicates that -unlike pollen, cosmogenic chronologies, or speleothems- the radiocarbon record of fossil moa remains provides an unbiased and precise indication of climate variability during the LGM and the LT. In my opinion this assumption may be flawed, as the link between the presence/absence of moa species and climate conditions is in fact quite indirect.
For instance, why moa remains are a better indicator of past vegetation than pollen assemblages?
Animals may change their diet specificity in response to climate alterations, and this could have certainly been the case for the moa species during the abrupt environmental changes while the world was thawing from the last glaciation. Hence, the changes in moa species during this time may be as an indirect climate proxy as pollen, isotopes, or other paleoclimate indicators.
Resolution of the moa radiocarbon sequences
While the author has made a great effort compiling a significant number of radiocarbon ages from moa fossil sites, some of the most critical inferences are based only on a small number of samples. For instance, the appearance of Pachyornis australis in the Takaka Hill site during the YD (indicative of cold/dry climates) is sustained just by two samples. Similarly, the responses to the Oruanui and Mt Takahe volcanic eruptions are inferred from a very limited number of radiocarbon dates.
Interpretation of the data
The current understanding derived from the literature is that the ACR was expressed in the Southern Hemisphere mid-latitudes either as a cooling interval or as a period with a slower warming rate. Furthermore, a significant increment in precipitation has been documented in west-facing continental areas as a result of an equatorial shift of the Southern Westerly Winds (SWW).
Such a SWW migration did more likely increase precipitation levels over most of western and southwestern New Zealand. Under this scenario:
Could the replacement of P. australis (cold/dry forest) by Anomalopteryx didiformis (warm/moist lowland forest) during the ACR have partially resulted from raising moisture levels rather than being a strictly warming signal?
Similarly, may the reappearance of P. australis during the YD chronozone respond to a sustained drying as the SWW shifted southward at the end of the ACR?
On the other hand, it is worth to note that the driver of the speleothem isotopic records has long been debated, with no current consensus that they wave a purely temperature signal. The author assumes that they represent genuine temperature records based on the similarities with the Greenland ice core isotope series. The author should discuss the potential driver and limitation of the NZ speleothem records, for which I suggest the following literature:
- Newnham, R.M., Vandergoes, M.J., Sikes, E., Carter, L., Wilmshurst, J.M., Lowe, D.J., McGlone, M.S. and Sandiford, A., 2012. Does the bipolar seesaw extend to the terrestrial southern mid-latitudes?. Quaternary Science Reviews, 36, pp.214-222.
- Barrell, D.J., Almond, P.C., Vandergoes, M.J., Lowe, D.J., Newnham, R.M. and INTIMATE members, 2013. A composite pollen-based stratotype for inter-regional evaluation of climatic events in New Zealand over the past 30,000 years (NZ-INTIMATE project). Quaternary Science Reviews, 74, pp.4-20.
Moa key morphological features
Although the systematics adopted by the author is explained in section 2.2, there is no discussion on which morphological features distinguish the different moa taxa discussed in the manuscript. This is an important element since the climate interpretation of the radiocarbon chronologies is based on the identification of different moa species. A brief discussion on the potential problems or limitations in the moa fossil identification would be desirable.
Excessive number of smoothed curves
Figure 2, 3, 4, 10 and 11 present all several different smoothing factors for the same dataset. Although the author has done a valuable effort to obtain the most representative long-frequency signal from the raw isotopic data, the great number of curves derived from the same original series makes the diagram hard to understand and, in many cases, respective. I would suggest choosing one smoothing factor, provide a justification for its selection, and use that single one for all the following figures. A diagram with all the different smoothing factors could be added in the supplementary information
Minor changes
Lines 31-32: Please provide age interval for the ACR.
Line 55-56: why Antarctic Ice core records were omitted?
Line 68: It is not necessary to include the annual sunshine hours.
Line 69: A reference is needed for “…the Holocene vegetation has been lowland rainforest…”.
Line 112-113: A reference is needed for “The Holocene TV rain forest was much floristically richer than that on TH.”.
Line 126: Merino Cave is not seen in Figure 1.
Line 210-214: All the different smoothing factors tested should be added in the supplementary information. For sake of simplicity and for a more straightforward reading of the text, only a single smoothed curve should be used in the figures.
Line 212-213: “At all SF values, the fall in temperature…” This statement is actually a result and should be moved to section 3.
Line 212-213: The fact that the cooling observed in all smoothing curves coincide with the YD as defined in Greenland records is just partially true. In William et al 2010, the lateglacial cooling is observed between13.4-11.2 kcal BP, so that it starts ~500 years earlier than the Northern Hemisphere YD.
Line 311: consider change “just preceding” by “prior to”.
Line 385-387: I challenge this statement. Moa chronologies are as indirect climate proxies as isotope or pollen timeseries. They do not record a direct temperature change, but rather changes in the moa habitat preferences, driven by vegetation change, which in turn respond to climate change.
Line 414: consider removing “both”.
Line 424: Please correct “Instead, therefore,”.
Figures and Tables
Figure 1: “Fossil deposits, black; speleothem sites, white”. I think it is actually the opposite. Please revise.
The site “A” Mt. Arthur (Nettlebed Cave) is not seen in the maps.
Figure 2: The author should consider simplifying this image with only a single selected smoothed factor on it. As all the different smoothing factors show the same pattern, the selection between them is not directly relevant for the topics discussed in the manuscript. Hence, the entire set of smoothing curves could be moved to the supplementary information.
Figure 5: A label for the YD interval (represented by a long vertical bar) is missing. A similar geometrical indicator for the ACR would be desirable.
Figure 8: Insert B seems not be necessary because the changes in the extent of the Cook Strait during the last termination are not discussed in the main text, nor the focus of the core discussion of the data.
Figure 9: Line 360- please indicate what δ18O record is used in Figure 9c.
Figure 11: This figure is basically the same as figure 9d, and therefore its presence on the main text is not strictly necessary.
Table 1: Why only the bold date was used for the sequence analysis?
Ignacio A. Jara, Center for Advanced Studies in Arid Zones, Chile
Citation: https://doi.org/10.5194/cp-2021-154-RC2 - AC2: 'Reply on RC2', Richard Holdaway, 17 Feb 2022
Status: closed
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RC1: 'Comment on cp-2021-154', Matt McGlone, 09 Jan 2022
General
This ms compiles and analyses previously published data on the occurrence of moa in cave deposits from the northern half of the South Island of New Zealand. On the basis of the assumed high habitat specificity of the moa species, and with reference to local d O18 speleothem records, a sequence of vegetation and climate change is deduced for the Late Pleistocene-early Holocene. In contrast to the developing consensus derived from New Zealand marine cores, glacial advances and pollen sequences from the South Island, it is claimed on basis of the moa dates that the Antarctic Cold Reversal (ACR: c. 14.5 – 12.8 kyr) was represented by a local warming and that the Younger Dryas (YD: 12.8-11.5 kyr) by a return to glacial conditions. While the author is to be congratulated on bringing together a comprehensive fossil moa data set to address the question of Late Pleistocene climate change, his failure to discuss or even cite the relatively large amount of detailed paleoclimatic data from this region which would lead to diametrically opposed conclusions makes his case untenable.
Specific comments
1Speleothem interpretation
The interpretation of the speleothem records in this paper assumes that they are entirely driven by local temperature change. This is a flawed assumption, as speleothem records are influenced by many factors ranging from distant oceanic moisture sources, rainout through to those affecting soil infiltration and precipitation in the cave system. “The immediate forcing factors for the observed palaeoclimatic variations are therefore most likely related mainly to changes in ocean source waters of precipitation, transmitted by westerly winds.” (Williams et al. 2010). As the late Pleistocene was a time of major changes in wind and oceanic water masses, it is highly likely that these rather than local temperature drove the changes. The current ms suggests that there was a warming between 14 and 13 ka followed by a cooling between 13 and 12 ka (Fig. 12). The interpretation given by those who generated the speleothem data does not entirely support this interpretation: “Late-glacial warming commenced between 18.2 and 17.8 ka and accelerated after 16.7 ka, culminating in a positive excursion between 14.70 and 13.53 ka. This was followed by a significant negative excursion between 13.53 and 11.14 ka of up to 0.55‰ depth that overlapped the Antarctic Cold Reversal (ACR) and spanned the Younger Dryas (YD). Positive δ18O excursions at 11.14 ka and 6.91–6.47 ka represent the warmest parts of the Holocene.” (Williams et al. 2005). Moreover, a careful examination of the curves in Williams et al. (2005) show that it was a period of warming, as those authors state: “We agree with the conclusion of Turney et al. that the YD chronozone in New Zealand was a period of resumed warming, because d18Oc values show an upwards trend after ~12.7 ka.”
2 Quality of the moa fossil record versus the pollen and macrofossil record
Some extraordinary claims are made in the ms on the value of the moa record versus the other palaeoclimatic indicators from the South Island and the surrounding ocean (lines 385-391):
“Unlike indirect proxies such as oxygen isotope ratios and pollen samples from a few sites, moa were on-ground real-time witnesses to the vegetation (and hence climate) at the time and place they lived and died. The congruence of the high spatial and temporal resolution record of vegetation based on 14 C ages of individual moa, each characteristic of a particular vegetation (Fig. 12) with the local speleothem δ18 O record suggest that both proxies accurately reflect the timing of changes in the New Zealand glacial and post-glacial climate. However, the postglacial return to cold climate in both records is later than the ACR. Instead, the moa sequence accords with the δ18 O record in showing that interval to be one of warming climate at these southern latitudes.” I make the following comments on these claims:
- The moa record is not particularly high resolution given that the dated remains are scattered through numerous cave systems.
- The argument that moa are a direct indicator of vegetation composition while pollen and macrofossils derived from that vegetation are indirect, is without merit. Moa like most large vertebrates would have used a variety of vegetation types and, rather than a documented analysis of the occurrence of moa in relation to vegetation, we have impressionistic claims. In contrast there is a long history in New Zealand of quantitative assessments of pollen rain versus vegetation type. Marine isotope and alkenone proxies likewise have an impressive, well understood, quantitative and physical basis.
- Interpolation of ages from dated horizons (which is critiqued here as inferior to inferences made on the basis of individual fossil dates as per the current ms) is a well established procedure and, although it can involve some inaccuracies, the mass of data now accumulated in the New Zealand region from dated cores is such that we can high confidence that the ACR was not a period of strong warming, whereas the YD was. Problems now largely centre not on the dating, but on the interpretation of how the various proxies react to climate forcing. For instance, some proxies represent a seasonal signal rather than an annual one.
3 Lack of discussion of other relevant climate proxies
The only local climate proxies discussed in detail here are dO18 speleothem records and moa fossils. I here briefly discuss some of the most relevant that are missing from such a discussion.
The results from glacier advance investigations are unequivocal (Doughty et al. 2013): “The modelling results presented in this paper supports the idea that the temperature during the ACR was 2-3 oC cooler than today in New Zealand. This temperature change estimate overlaps, within error, with a temperature change estimate of 3-4 oC cooler than today, reported by Anderson and Mackintosh (2006) for the Franz Josef Glacier advance to the Waiho Loop during the Lateglacial. Our results are also consistent with published pollen studies (Newnham and Lowe, 2000; Turney et al., 2003; Vandergoes and Fitzsimons, 2003; Hajdas et al., 2006), and regional sea-surface temperature records (Carter and Cortese, 2009; Sikes et al., 2009) that suggest a temporary reversal of the deglaciation warming trend occurred in New Zealand during the Lateglacial”
Marine core palaeoclimatic analyses directly to the west of the northern South Island also support the concept of an ACR cool period and strong warming during the YD: “There are no short-term fluctuations late in the Pleistocene that could potentially be a YD cooling event, even allowing for generous errors in the age model. Consistent with warming in the SST series, the YD interval appears to be associated locally with a period of maximum forest growth (increasing abundance of podocarps and hardwoods, Fig. 1), not cooling.” (Barrows et al. 2007).
A detailed pollen and macrofossil study (Jara et al. 2015) near treeline in Northwest Nelson, close to the moa caves that provide most of evidence in the current ms, also does not support the interpretation of a warm ACR and a cold YD. Temperatures modelled on the basis of pollen from Adelaide Tarn are well below current during the ACR, whereas the YD chronozone sees a rapid rise in temperature to above present day levels. On the central-eastern flank of the Southern Alps, pollen and macrofossil evidence point to a cool ACR and strong warming during the YD (McGlone et al. 2004).
Further south to the east of the South Island, marine core evidence (Pahnke et al. 2006; Pahnke, 2003) indicates continuous warming from 18 ka, but with a steep acceleration during the YD chronozone. The subantarctic Campbell Island pollen record immediately to the south of the South Island shows cool conditions during the ACR followed by steep warming during the YD (McGlone et al. 2010, McGlone, et al. 2019).
Concluding remarks
Overturning a broad consensus (based on numerous New Zealand late Pleistocene-early Holocene investigations of glacial moraines, deep sea cores and pollen and macrofossil cores) that the ACR in this region of the southwest Pacific was either cool or warming at a slower rate than the subsequent rapid temperature increase during the YD, requires much more substantial evidence than that presented here. The speleothem evidence cannot be attributed directly to temperate alone, and the most parsimonious explanation for the mis-match of the dO18 results from this source with other proxies is changing oceanic sources and wind flow. Moreover, even if it could be demonstrated that the moa fossils are reliable indicators of changing vegetation, given the low temporal resolution of the evidence presented and the fact that these were large, highly mobile animals, this evidence does not undermine the solid, well replicated, well dated paleotemperature records derived from moraines, deep-sea cores and pollen and macrofossils from adjacent South Island sites. Some reconciliation would be needed if the re-interpretation suggested were to be accepted, but this current ms barely recognizes the existence of this data, let alone attempts to discuss it.
I therefore see no reason based on the results in this ms to abandon the concept that temperature trends during the late Pleistocene-early Holocene in the New Zealand region were roughly in antiphase with those in the mid to high latitudes of Northern Hemisphere.
Matt McGlone, Manaaki Whenua-Landcare Research, Lincoln, New Zealand
Barrows, T. T., et al. (2007). "Absence of cooling in New Zealand and the adjacent ocean during the Younger Dryas chronozone." Science 318 (5847): 86-89.
Doughty, A. M., et al. (2013). "Evaluation of lateglacial temperatures in the Southern Alps of New Zealand based on glacier modelling at Irishman Stream, Ben Ohau Range." Quaternary Science Reviews 74: 160-169.
Jara, I. A., et al. (2015). "Pollen-climate reconstruction from northern South Island, New Zealand ( 41 degrees S), reveals varying high- and low-latitude teleconnections over the last 16 000 years." Journal of Quaternary Science 30(8): 817-829.
McGlone, M. S., et al. (2004). "Late-glacial and Holocene vegetation and climatic history of the Cass basin, central south island, New Zealand." Quaternary Research 62(3): 267-279.
McGlone, M. S., et al. (2010). "Divergent trends in land and ocean temperature in the Southern Ocean over the past 18,000 years." Nature Geoscience 3(9): 622-626.
McGlone, M. S., et al. (2019). "Temperature, wind, cloud, and the postglacial tree line history of sub-Antarctic Campbell Island." Forests 10(11): 998.
Pahnke, K. & Sachs, J. P. 2006. Sea surface temperatures of southern midlatitudes 0–160 kyr BP. Paleoceanography 21, PA2003. 5.
Pahnke, K., Zahn, R., Elderfield, H. & Schulz, M. 2003. 340,000-year centennial-scale marine record of Southern Hemisphere climatic oscillation. Science 301, 948–952
Williams, P. W., et al. (2005). "Late Pleistocene to Holocene composite speleothem 18O and 13C chronologies from South Island, New Zealand—did a global Younger Dryas really exist?" Earth and Planetary Science Letters 230(3-4): 301-317.
Williams, P. W., et al. (2010). "Age frequency distribution and revised stable isotope curves for New Zealand speleothems: palaeoclimatic implications." International Journal of Speleology 39(2): 99-112.
Citation: https://doi.org/10.5194/cp-2021-154-RC1 - AC1: 'Reply on RC1', Richard Holdaway, 16 Feb 2022
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RC2: 'Comment on cp-2021-154', Ignacio Jara, 10 Feb 2022
Overview
The author presents a series of radiocarbon chronologies of fossil moa remains covering the Last Glacial Maximum (LGM) and the Last Termination (LT) from several fossil sites in the South Island of New Zealand. The climate interpretation of the 14C sequences assumes that moa species are in strict association with specific vegetation types; and uses New Zealand’s δ18O speleothem series as a master temperature record. The moa chronologies are then evaluated in the light of long-lasting debate about temperature changes during lateglacial times in New Zealand, coming up with a rather controversial claim of rapid warming during the Antarctic Cold Reversal (ACR; 14.6-13 kcal BP) and significant cooling aligned with the Younger Dryas chronozone (YD; 12.9-11.7 kcal BP).
Although the use of habitat-specialist moa species as a proxy for temperature change is definitely a novel approach, and even though the author have made a comprehensive revision of the published radiocarbon chronologies; the manuscript does not seem to provide the sufficient evidence and the necessary revision of the vast literature to sustain some of its main conclusions.
For instance, the author argues than moa fossil assemblages are a more direct climate proxy than pollen or isotopes records, yet this hard to justify considering that the moa’s habitat specificity could have certainly changed in the face of rapid landscape changes, and that the chronologies presented in the manuscript represent a discontinuous record of individual, ephemeral paleoecological events.
The results presented by the author challenge the present model of inter-hemispheric relationship between climate events during the LT, and therefore hold large implications for the Quaternary paleoclimate community. To sustain such a big claim, a convincing explanation about why significant warming (cooling) occurred in NZ during the ACR (YD) at the same time than several other mid-latitude terrestrial records were documenting significant cooling (warming) is essential. Unfortunately, it is missing in the text.
Specific comments
Lack of a hemispheric perspective
The results presented in the manuscript challenge the current consensus about the climate events that characterised LT in the Southern Hemisphere. In this regard, the author fails to place the NZ moa chronology within a continental paleoclimatic context. There is solid evidence, from a good number of high resolution, well-dated climate records, to sustain an extension of the Antarctic deglaciation pattern into the Southern Hemisphere mid-latitudes; however, only a very small number of studies are mentioned or discussed in the main text.
I suggest the inclusion and thorough revision of the some of the most recent literature concerning this topic, included but not limited to:
- Fletcher, M.S., Pedro, J., Hall, T., Mariani, M., Alexander, J.A., Beck, K., Blaauw, M., Hodgson, D.A., Heijnis, H., Gadd, P.S. and Lise-Pronovost, A., 2021. Northward shift of the southern westerlies during the Antarctic Cold Reversal. Quaternary Science Reviews, 271, p.107189.
- Mendelova, M., Hein, A.S., Rodes, A., Smedley, R.K. and Xu, S., 2020. Glacier expansion in central Patagonia during the Antarctic Cold Reversal followed by retreat and stabilisation during the Younger Dryas. Quaternary Science Reviews, 227, p.106047.
- Sagredo, E.A., Kaplan, M.R., Araya, P.S., Lowell, T.V., Aravena, J.C., Moreno, P.I., Kelly, M.A. and Schaefer, J.M., 2018. Trans-pacific glacial response to the Antarctic Cold Reversal in the southern mid-latitudes. Quaternary Science Reviews, 188, pp.160-166.
- Hinojosa, J.L., Moy, C.M., Vandergoes, M., Feakins, S.J. and Sessions, A.L., 2019. Hydrologic change in New Zealand during the last deglaciation linked to reorganization of the Southern Hemisphere westerly winds. Paleoceanography and Paleoclimatology, 34(12), pp.2158-2170.
- Moreno, P.I., Denton, G.H., Moreno, H., Lowell, T.V., Putnam, A.E. and Kaplan, M.R., 2015. Radiocarbon chronology of the last glacial maximum and its termination in northwestern Patagonia. Quaternary Science Reviews, 122, pp.233-249.
As the evidence presented in this manuscript is not discussed in the light of these (and several others) studies, the author omits an explanation for why the Moa chronologies suggest a warming-cooling pattern during the ACR-YD intervals, whilst terrestrial records from other southern mid-latitudes regions indicates the opposite climate pattern.
In addition, no Antarctic ice core data has been included in the figures or discussed in the main text. It is rather surprising that the manuscript places so much attention to the Greenland ice core data without mentioning or discussing the detailed Antarctic ice core isotope or gas timeseries. It seems that the author is over-stressing the data that agrees with its interpretation of the moa sequences (i.e., NZ speleothems, Greenland ice cores), to the detriment of a great number of detailed and well-dated records from NZ, the mid-latitudes, and Antarctica.
Radiocarbon sequence of moa as a proxy for hemispheric climate events
The author indicates that -unlike pollen, cosmogenic chronologies, or speleothems- the radiocarbon record of fossil moa remains provides an unbiased and precise indication of climate variability during the LGM and the LT. In my opinion this assumption may be flawed, as the link between the presence/absence of moa species and climate conditions is in fact quite indirect.
For instance, why moa remains are a better indicator of past vegetation than pollen assemblages?
Animals may change their diet specificity in response to climate alterations, and this could have certainly been the case for the moa species during the abrupt environmental changes while the world was thawing from the last glaciation. Hence, the changes in moa species during this time may be as an indirect climate proxy as pollen, isotopes, or other paleoclimate indicators.
Resolution of the moa radiocarbon sequences
While the author has made a great effort compiling a significant number of radiocarbon ages from moa fossil sites, some of the most critical inferences are based only on a small number of samples. For instance, the appearance of Pachyornis australis in the Takaka Hill site during the YD (indicative of cold/dry climates) is sustained just by two samples. Similarly, the responses to the Oruanui and Mt Takahe volcanic eruptions are inferred from a very limited number of radiocarbon dates.
Interpretation of the data
The current understanding derived from the literature is that the ACR was expressed in the Southern Hemisphere mid-latitudes either as a cooling interval or as a period with a slower warming rate. Furthermore, a significant increment in precipitation has been documented in west-facing continental areas as a result of an equatorial shift of the Southern Westerly Winds (SWW).
Such a SWW migration did more likely increase precipitation levels over most of western and southwestern New Zealand. Under this scenario:
Could the replacement of P. australis (cold/dry forest) by Anomalopteryx didiformis (warm/moist lowland forest) during the ACR have partially resulted from raising moisture levels rather than being a strictly warming signal?
Similarly, may the reappearance of P. australis during the YD chronozone respond to a sustained drying as the SWW shifted southward at the end of the ACR?
On the other hand, it is worth to note that the driver of the speleothem isotopic records has long been debated, with no current consensus that they wave a purely temperature signal. The author assumes that they represent genuine temperature records based on the similarities with the Greenland ice core isotope series. The author should discuss the potential driver and limitation of the NZ speleothem records, for which I suggest the following literature:
- Newnham, R.M., Vandergoes, M.J., Sikes, E., Carter, L., Wilmshurst, J.M., Lowe, D.J., McGlone, M.S. and Sandiford, A., 2012. Does the bipolar seesaw extend to the terrestrial southern mid-latitudes?. Quaternary Science Reviews, 36, pp.214-222.
- Barrell, D.J., Almond, P.C., Vandergoes, M.J., Lowe, D.J., Newnham, R.M. and INTIMATE members, 2013. A composite pollen-based stratotype for inter-regional evaluation of climatic events in New Zealand over the past 30,000 years (NZ-INTIMATE project). Quaternary Science Reviews, 74, pp.4-20.
Moa key morphological features
Although the systematics adopted by the author is explained in section 2.2, there is no discussion on which morphological features distinguish the different moa taxa discussed in the manuscript. This is an important element since the climate interpretation of the radiocarbon chronologies is based on the identification of different moa species. A brief discussion on the potential problems or limitations in the moa fossil identification would be desirable.
Excessive number of smoothed curves
Figure 2, 3, 4, 10 and 11 present all several different smoothing factors for the same dataset. Although the author has done a valuable effort to obtain the most representative long-frequency signal from the raw isotopic data, the great number of curves derived from the same original series makes the diagram hard to understand and, in many cases, respective. I would suggest choosing one smoothing factor, provide a justification for its selection, and use that single one for all the following figures. A diagram with all the different smoothing factors could be added in the supplementary information
Minor changes
Lines 31-32: Please provide age interval for the ACR.
Line 55-56: why Antarctic Ice core records were omitted?
Line 68: It is not necessary to include the annual sunshine hours.
Line 69: A reference is needed for “…the Holocene vegetation has been lowland rainforest…”.
Line 112-113: A reference is needed for “The Holocene TV rain forest was much floristically richer than that on TH.”.
Line 126: Merino Cave is not seen in Figure 1.
Line 210-214: All the different smoothing factors tested should be added in the supplementary information. For sake of simplicity and for a more straightforward reading of the text, only a single smoothed curve should be used in the figures.
Line 212-213: “At all SF values, the fall in temperature…” This statement is actually a result and should be moved to section 3.
Line 212-213: The fact that the cooling observed in all smoothing curves coincide with the YD as defined in Greenland records is just partially true. In William et al 2010, the lateglacial cooling is observed between13.4-11.2 kcal BP, so that it starts ~500 years earlier than the Northern Hemisphere YD.
Line 311: consider change “just preceding” by “prior to”.
Line 385-387: I challenge this statement. Moa chronologies are as indirect climate proxies as isotope or pollen timeseries. They do not record a direct temperature change, but rather changes in the moa habitat preferences, driven by vegetation change, which in turn respond to climate change.
Line 414: consider removing “both”.
Line 424: Please correct “Instead, therefore,”.
Figures and Tables
Figure 1: “Fossil deposits, black; speleothem sites, white”. I think it is actually the opposite. Please revise.
The site “A” Mt. Arthur (Nettlebed Cave) is not seen in the maps.
Figure 2: The author should consider simplifying this image with only a single selected smoothed factor on it. As all the different smoothing factors show the same pattern, the selection between them is not directly relevant for the topics discussed in the manuscript. Hence, the entire set of smoothing curves could be moved to the supplementary information.
Figure 5: A label for the YD interval (represented by a long vertical bar) is missing. A similar geometrical indicator for the ACR would be desirable.
Figure 8: Insert B seems not be necessary because the changes in the extent of the Cook Strait during the last termination are not discussed in the main text, nor the focus of the core discussion of the data.
Figure 9: Line 360- please indicate what δ18O record is used in Figure 9c.
Figure 11: This figure is basically the same as figure 9d, and therefore its presence on the main text is not strictly necessary.
Table 1: Why only the bold date was used for the sequence analysis?
Ignacio A. Jara, Center for Advanced Studies in Arid Zones, Chile
Citation: https://doi.org/10.5194/cp-2021-154-RC2 - AC2: 'Reply on RC2', Richard Holdaway, 17 Feb 2022
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