Tuatara: Volume 16, Issue 1, April 1968
Molluscan Evidence for Tertiary sea temperatures in New Zealand: a reconsideration
Molluscan Evidence for Tertiary sea temperatures in New Zealand: a reconsideration
Numbers of Indo-Pacific molluscan genera occurring in each stage of the New Zealand Tertiary are totalled in three ways: (1) genera assumed to have been present from their first to their last occurrences; (2) the actual recorded occurrences of genera; (3) genera occurring only in the North Island. These are interpreted from evidence of knowledge of the temperature ranges of genera, and of revised ages for some Northland fossil localities.
The temperature graph produced for the South Island suggests that marginally tropical conditions prevailed in the late Arnold Epoch, late Landon Epoch and early Southland Epoch, with slightly cooler intervening periods, and that temperatures rose gradually until the first peak and fell gradually after the last peak. A few scattered points on a graph for Northland suggest that the area was marginally tropical from at least Bortonian to Waiauan times.
One of us (Beu, 1966) recently discussed the molluscan evidence for sea temperatures in New Zealand during the Cenozoic Era, and concluded that temperatures rose evenly through the Tertiary Period to a peak in the Otaian (Lower Miocene), then decreased gradually through the rest of the period. Temperatures at the end of the Pliocene were considered to have been only a little warmer than those of the present day.
Fig. 1: Numbers of Indo-Pacific molluscan genera in New Zealand Tertiary stages. Black histogram shows numbers recorded from South Island localities; cross-hatched histogram shows numbers recorded in the North Island only; uppermost (unshaded) histogram shows numbers present in each stage, assuming each genus was present from its first until its last recorded occurrence.
Factors which were not considered by the writers in this survey include the effects of relative displacement along the alpine fault during Tertiary time, and possible temperature differences between the east and west coasts of New Zealand.page 71
Methods Used in Deducing Temperatures
The writers had access to charts, prepared by Dr. C. A. Fleming for his Pacific Science Congress paper (Fleming, 1967), showing the stratigraphic ranges of all New Zealand Cenozoic molluscan genera, classified into eight biogeographic categories. A list of Indo-Pacific genera and their actual recorded occurrences in New Zealand was prepared from these, incorporating new records and some reclassifications, and taking into account the age revisions mentioned above. This gave a total of 168 genera.
Genera were assumed to have been present continuously from their first until their last occurrences;
The actual recorded occurrences of all genera throughout New Zealand were totalled for each stage;
Genera occurring only in the North Island were segregated from those in the South Island again using actual occurrences.
Total ranges: A gradual rise through the Tertiary until the Hutchinsonian, and a gradual fall through the rest of the Tertiary;
Actual occurrences: A far less regular histogram, with marked minima in the Runangan, Kapitean and Waitotaran, and maxima in the Kaiatan, Otaian and Hutchinsonian, Tongaporutuan and Opoitan;
Removing the North Island element, and assuming that Indo-Pacific genera occurring in the South Island were also present in the North Island: also irregular, with minima in the Runangan, Otaian and Hutchinsonian. Waitotaran, and a minor one in the Tongaporutuan and Kapitean. Maxima occur in the Kaiatan, Duntroonian and Waitakian, Awamoan, and a minor one in the Opoitian.
The histograms cannot be interpreted directly as temperature graphs. For example, the small number of records in the Runangan and Whaingaroan is due to the lack of knowledge of faunas of this age. Construction of a temperature graph from the histograms involves considerable re-interpretation.
One approach is to consider the proportion of Indo-Pacific genera in the total molluscan fauna of each stage. The writers calculated these as percentages, using actual recorded occurrences for the whole country, and plotted them as a histogram (Figure 2). This has the effect of inverting the minimum in the Runangan and maximum in the Opoitian and Waipipian. A marked maximum still remains in the Hutchinsonian, but the Otaian and Awamoan are relatively low. A gradual rise from Awamoan until Clifdenian times seems page 73 significant, particularly as almost all records in these stages are from the South Island.
A second approach is to consider the changes of geographic range with time of stenothermal warm-water genera in the New Zealand Tertiary. Actual recorded occurrences of the most significant 21 of these are shown in Figure 3. Such genera were almost entirely restricted to Northland during the Pareora Epoch, and migrated south by Altonian-Clifdenian times.
The Temperature Graph
The temperature graph shown in Figure 4 was constructed from evidence of the sort outlined above. The graph shows that sea temperatures in the South Island were marginally tropical in the late Arnold Epoch, late Landon Epoch and early Southland Epoch, was slightly cooler intervening periods, and that temperatures rose gradually until the first peak and fell away gradually after the last peak. All molluscan evidence in Northland suggests that temperatures were marginally tropical from at least Bortonian to Waiauan times, although the evidence is scanty for the time prior to the Otaian and after the Awamoan.
We are grateful to Dr. C. A. Fleming for the use of his range charts of New Zealand Cenozoic Mollusca, and of the manuscript and proofs of his paper presented in Japan. We also wish to thank Mr. G. H. Scott for information on revised ages of the sequence at Pakaurangi Point, and Professor P. Vella of the Geology Department, Victoria University of Wellington and Dr. Fleming for critically reviewing the manuscript.
Dr. N. deB. Hornibrook. In the distribution of the large Foraminiferia during the middle Tertiary there is a period in the Landon when they are practically all absent or at least very scarce. Do you have such a picture with the Mollusca — an absence of nearly all Indo-Pacific species?
Mr. P. A. Maxwell. Apart from an apparent decrease in the number of Indo-Pacific genera in the Waingaroan, which may be due to the paucity of good macrofaunas, there is really no dramatic change. The Duntroonian for instance has quite a high proportion of Indo-Pacific molluscan genera.
Dr. P. Webb. In the Paleocene have you compared the faunas to those in Victoria such as the Pebble Point faunas, and have you noticed any Paleocene affinities with places like Chile or other South Pacific areas?page 74
Mr. P. A. MAXWELL. The Victorian Paleocene molluscan fauna is not very large as I recall but does include the bivalve Lahilleona also known from the New Zealand Upper Cretaceous and Wangaloan. The closely related Lahillia occurs in Patagonia. A possible link between New Zealand and Patagonia is the gastropod Zealandiella doubtfully recorded from the Wangaloan which may be the same as a South American genus Austrocominella but I am not sure of the stratigraphical distribution of the latter.
Mr. K. B. Lewis. Do you get any influxes of Antarctic forms during the Tertiary such as during the Waipipian. or are there just influxes and extinctions of warm water forms?
Mr. P. A. Maxwell. I would not like to say definitely that any of the Tertiary species were Antarctic forms. There are some which come in during the Pleistocene. The Waipipian has a decidedly warm water aspect so far as the Mollusca are concerned.
Mr. I. Devereux. You have some very small bumps on your curve of only two degrees. Do you think that you can detect such a change or were you being rather conservative?
Mr. P. A. Maxwell. It is very hard to decide the scale of these bumps. They could easily be larger. They are plotted more to give the position of some warming rather than the amount.
Dr. C. A. Fleming. Would you say that the end of the Tertiary was cooler than today?
Mr. P. A. Maxwell. Yes, but only a little. About the same as the South Island today, or Wellington.
Beu. A. G., 1966. Sea temperatures in New Zealand during the Cenozoic Era, as indicated by molluscs. Trans, roy. Soc. N.Z. Geol. 4 (9): 177-187, 1 fig.
Garner, D. M., and Ridgeway, N. M., 1965. Hydrology of New Zealand Offshore Waters. N.Z. oceanogr. Inst. Mem. 12: 62 pp., 55 figs., 2 maps.
Fleming, C. A., 1963. The nomenclature of biogeographic elements in the New Zealand Biota. Trans. roy. Soc. N.Z. General 1 (2): 13-22.
—— 1967. Cenozøic history of Indo-Pacific elements in the marine fauna of New Zealand. Venus 25: 105-117.
* Geology Department, Victoria University of Wellington; present address, New Zealand Geological Survey, Lower Hutt.
† New Zealand Geological Survey, Lower Hutt.