Abstract

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.

Introduction

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.

Fleming (1967) in a paper presented at the 11th Pacific Science Congress. Tokyo, 1966, listed all genera of known or assumed Indo-Pacific affinities occurring in the New Zealand Cenozoic, together with their stratigraphic ranges. Criteria for recognising Indo-Pacific genera were given by Fleming (1963, 1967). Fleming 1967; page 69

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.

text-fig. 4) gave a histogram showing total numbers of Indo-Pacific molluscan genera in each stage of the New Zealand Cenozoic, assuming that genera were present continuously from their first until their last occurrences. If interpreted as a temperature graph, this resembles the one given by Beu (1966: fig. 1), reaching a maximum in the Lower Miocene.

Fleming and Beu assumed that the beds at Pakaurangi Point. Kaipara Harbour, were entirely of Otaian age. Recent work by Mr. G. H. Scott (pers. comm) has shown that the greater part of the Pakaurangi Point section is Hutchinsonian in age, and only the page 70

Fig. 2: dido-Pacific molluscan genera recorded from each stage of the New Zealand Tertiary, expressed as a percentage of the total recorded molluscan fauna.

lowest beds are late Otaian. Neither author allowed for latitudinal temperature variation over New Zealand, and both produced a generalised graph for the warmest faunas known in each stage throughout the country. Maps of isotherms given by hydrologists such as Garner and Ridgeway (1965) show that surface temperatures at Stewart Island and at North Cape differ by approximately five to eight degrees at the present day, and it seems reasonable to infer that similar latitudinal differences existed in the past. This is important in considering New Zealand Tertiary temperatures, as the richest molluscan faunas of Paleocene to Miocene age are situated in the southern half of the South Island, whereas many warm-water genera are known only from Lower Miocene localities in Northland. The writers have therefore attempted to draw separate graphs for the southern part of the South Island and for Northland.

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.

Fig. 3: Geographic and stratigraphic distribution in the New Zealand Tertiary of 21 significant stenothermal warm-water molluscan genera. Broad line indicates occurrence in the South Island (and presumably in the North Island also); narrow line indicates occurrence in the North Island only.

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.

Fig. 4: Sea temperatures in New Zealand during the Tertiary Period, from molluscan evidence. Lower curve shows South Island temperatures; upper curve shows temperatures in Northland.

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.

Acknowledgements

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.

Discussion

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?

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.