Abstract

Extinction of orbitoidal foraminifera was followed by regional transgression and by an invasion of the foraminiferid Bolivinita quadrilatera in New Zealand (40°South) Indonesia (equatorial) and Japan (40°North). As the simplest hypothesis it is assumed that all three events happened in the three areas and throughout the western Pacific at the same time. The most likely cause of all three events over such a large area would be a worldwide temperature fall and rise with a minimum more than 5°C lower than late Middle Miocene temperature.

Introduction

In 1963 I postulated a glacially controlled fluctuation of sea-level to explain a sedimentary cycle (Te Aute Cyclothem) in strata classed as Upper Pliocene (Waitotaran Stage). I also suggested that waxing and waning of an Antarctic Ice cap might have caused sea-level fluctuations during the Tertiary Period. Recently Kennett (1967, p.1061) postulated a cooling in New Zealand during latest Miocene (Kapitean) time on the evidence of changes in planktonic foraminiferal faunas and the occurrence of dominantly sinistral-coiled populations of Globigerina (or Globorotalia) pachyderma. He drew attention to evidence of a fall in sea-level and faunal changes that happened at about the same time in other parts of the world, postulated that the fall in temperature was world-wide, and estimated it to be about equal to those of the Pleistocene glacial periods. Jenkins (1967) described changes in coiling ratios of Globigerina pachyderma samples from Upper Miocene to Pleistocene in New Zealand, and inferred a series of temperature fluctuations during the Pliocene Period.

Middle to Upper Miocene stratigraphy and paleontology

Basal Upper Miocene (Tongaporutuan) strata known as the Hurupi Formation overlie older rocks with angular unconformity along an outcrop of about 120 kilometers in Wairarapa, East Wellington. Where microfaunas have been examined the lowest 50 to 100 meters of the formation contains shallow-water foraminifera whose age could be Middle or Upper Miocene, but higher beds contain Bolivinita quadrilatera the first appearance of which marks the base of the Tongaporutuan Stage throughout New Zealand. The basal beds of the Hurupi Formation seem to be virtually the same age throughout its 120 meters of known outcrop. A sandstone at the base of the Waihoki Series (Ongley, 1935) which unconformably overlies Middle Miocene at Pongaroa, is probably the north-eastward continuation of the Hurupi Formation, but its age is not yet well established.

The Waiauan Stage which directly underlies the Tongaporutuan in New Zealand; is generally accepted as the highest part of the Middle Miocene. It is marked locally by the last appearance of orbitoid foraminifera in New Zealand, and consequently Hornibrook (in Fleming ed. 1959, p. 429) correlated it with the upper part of the f Stage (f3) of Indonesia.

A regional unconformity marks the base of the g Stage over large parts of Indonesia, but tends to fade out where sediments are thick (van Bemmelen, 1949, p. 88 and many tables and diagrams). The g Stage directly overlies the f Stage and is regarded as representing the Upper Miocene and part of the Pliocene. Van Bemmelen attributed the unconformity to a phase of diastrophism during the Middle Miocene Epoch. The boundary between the g Stage and underlying f Stage is paleontologically marked by the sudden and total disappearance of orbitoid foraminifera which are abundant and varied in f3, the youngest substage of the f Stage (Rutten, in van Bemmelen, 1949, p. 83-3). It is also marked by the first appearance or nearly the first appearance of Bolivinita quadrilatera (Finlay, 1947; Glaessner, 1959).

At Kakegawa in Honshu, Japan, the Upper Miocene Sagara Group rests unconformably on strata correlated with the Burdigalian Stage (Lower Miocene) of Europe, and in the Izu Peninsula nearby strata immediately older than the Sagara Group contains Bolivinita quadrilatera, and because it lacks orbitoid foraminifera is correlated with the g Stage of Indonesia (Sawai, 1962, p. 116).

Inferred temperature fluctuations between Middle and Upper Miocene, and estimate of its magnitude

A widespread transgression was accompanied by the invasion of Bolivinita quadrilatera and preceded by the extinction of orbitoid page 47 foraminifera in each of the three regions discussed. It is most likely that the three events resulted from one basic cause or combination of causes, and the simplest assumption is that they happened simultaneously in the three regions.

The most likely cause of the extinction of the orbitoid foraminifera, and one which would affect the three regions simultaneously, is a world-wide drop in temperature. Japan and New Zealand straddle the 40 degrees North and 40 degrees South latitudes respectively. The late Miocene orbitoid faunas of these two countries are localised and are probably mere relicts of former more prolific orbitoid faunas. Probably they were living near the limit of their temperature tolerance and would have been extinguished by a slight temperature decrease. A much larger temperature decrease would be needed to extinguish the abundant and varied equatorial fauna of Indonesia. As a first approximation the magnitude of the decrease needed may be taken as greater than the difference between the temperature at the equator and that at the 40 degrees latitudes represented by Japan and New Zealand where we have assumed that the Middle Miocene orbitoids were at the limit of their tolerance. It was probably of the same order as the 10°C to 12°C difference in mean annual temperature of surface waters in New Zealand and Indonesia at the present day. Even if it is assumed that climatic zones were less strongly differentiated than now the estimated temperature difference can scarcely be reduced to less than half that of the present day, and we may safely assume a minimum possible difference of 5°C. A temperature drop of this magnitude or greater could be expected to cause a significant growth of a polar ice cap and hence a fall in sea-level.

A decline in temperature of 5°C or more is not consistent with differences in fossil faunas of the Middle Miocene and those of most of the Upper Miocene in New Zealand (Hornibrook, in Fleming ed. 1959, p. 407). Therefore it is necessary to postulate that the greater part of the large drop in temperature at the end of the Middle Miocene Epoch was temporary, and that early in the Upper Miocene Epoch temperature recovered to something approaching that of the Middle Miocene. The recovery of temperature would cause retreat of polar ice and rise of sea-level.

While temperature was falling all tectonically sinking land areas would remain above sea-level except those sinking faster than sea-level. On the contrary, when sea-level began to rise all sinking land areas would be flooded extremely swiftly, as was Wairarapa at the beginning of the Upper Miocene Epoch.

Conclusions

It is postulated that a world wide decrease in temperature greater than 5°C possibly as much as 12°C took place at the end of the Middle Miocene Epoch and directly caused the extinction of orbitoid foraminifera (and other organisms) in the western Pacific and probably also throughout the world. It is also suggested that the decrease in temperature caused glacial lowering of sea-level.

It is further postulated that temperature rose again during the early part of the Upper Miocene Epoch, causing a post-glacial rise of sea-level which is reflected as transgression in New Zealand, Indonesia and Japan. With the transgression characteristic Upper Miocene species including Bolivinita quarilatera invaded the three countries.

Discussion

Dr. N. deB. HORNIBROOK. I wonder if Dr. Squires could comment on any evidence in deep cores in the Pacific from say Bikini, to support this large fluctuation in temperature.

Dr. D. F. Squires. I don't think there is any such evidence. I am wondering if there has been undue emphasis on a temperature effect. Possibly a smaller change would have affected the food supply of the foraminifera; for instance the seaweed.

Professor P. Vella. The temperature change I have suggested is somewhat similar to that shown by Mr. Devereux for his drop from the Southland Series to the Lower Tongaporutuan.

Dr. C. A. Fleming. If such a large change in temperature seems unreasonable possibly some change such as proposed by Professor Wilson may be the explanation. A test for Professor Vella's hypothesis would be to closely examine a section continuous from Waiauan to Tongaporutuan.

Professor P. Vella. This may be helpful but care must be taken as tectonic movements could lead to quite erroneous conclusions.