Has New Zealand recently entered an entirely new period of time?
You may have caught up with the proposal that the world is now in a new era of time – the Anthropocene. Geologists have previously told us that we lived in the period called the Holocene. This is generally agreed to have begun 11,700 years ago – as the world rapidly warmed out of the last glacial period. When I first heard the term ‘Anthropocene’ I assumed it was more a smart marketing ploy to draw attention to a lot of the world’s serious issues. Actually, it is partially that, but that would be to also miss the point.
Geological divisions of time are based on recognising periods of geologically sudden, profound, and regionally recognisable change in the sedimentary record. These become codified as the boundaries between time periods. Several of the major periods in Earth history are recognised primarily through pervasive extinction events. The Cretaceous-Cenozoic boundary, when the dinosaurs became extinct, is one, extreme, example. But less pronounced change can also be the basis of period boundaries and also, looking beyond fossils, boundaries may also be recognised on other criteria – for example chemistry or climate.
Recognising whether we have recently entered a new time period requires that we take a geological perspective. If we were geologists, say, 100 million years from now, and set to defining a geological time scale – would we recognise an important boundary for about now? For the people responsible for ‘managing’ the current international scale, (Stratigraphy Commission of the Geological Society of London) the answer is definitely “Yes”. My question here is, if geologists 100 million years ago were only looking at New Zealand – would they come to the same conclusion?
When they study fossils, future geologists will find a remarkable extinction event correlating with the appearance of humans in New Zealand. Humans arrived in New Zealand about AD 1200-1400 (McGlone and Wilmshurst, 1999). These were Polynesians, and were followed 600 years or less later by Europeans. This 600 years, which is of so much importance to historians and politicians, is utterly trivial on geological time scales. Six hundred years would be well below the level of resolution of all but the slowest and most continuous sedimentary successions. For the geologists 100 million years in the future, it would be essentially instantaneous.
Over that ‘boundary’ future paleontologists would note that New Zealand lost 40-50% of its birds (Holdaway 1989). This is the proportion of birds that have become extinct in New Zealand since humans arrived – it doesn’t include those species that have had their range reduced so much that realistically, they will no longer contribute to a fossil record, and may yet go completely extinct. A similar amount of extinction has hit the frog fauna (Holdaway 1989), while the figures are still unclear for other groups such as lizards and insects. At the same time, there has been a massive influx of new species from beyond New Zealand, including entirely new groups of land verterbrates, and of course, humans themselves.
One of the surest signs of humans has been the dramatic increase in charcoal content of sediments from almost nil. Palynologists often study the charcoal content of peat swamps along with the enclosed pollen. In New Zealand this kind of study shows the increase of charcoal starting around 800 years ago McWethy et al. 2009, 2010). A hundred million years from now, what peat remains will be coal. The typical one or two meter column of peat that palynologists study today might then be only 30 cm or so thick. Furthermore, what was a nice sampling interval in the peat (to get enough resolution to understand the process) – say two centimetres, will now be compacted to just millimetres, and very difficult to study. Once again, on a geological scale, the appearance of abundant charcoal in New Zealand will appear practically instantaneous. It will correlate with the appearance of diverse range of pollen from introduced plants.
Beyond their actual artifacts, humans are making other changes that will be apparent in the future geological record. Deforestation (some of it the result of the afore-mentioned burning) have significantly increased erosion rates (McSaveny and Whitehouse 1989). The corollary of this is increased sedimentation somewhere else. River, lakes, estuaries are silting up. After a long period of fairly stable Holocene sea level, sea level is on the rise due to global warming. At around 3 mm per year, it seems so slow. But at 3 m in 10,000 years, or 30 m in 100, 000 years, this will ultimately be recognisable as a transgression – with a ‘sequence boundary’ at its base. A change in the character of New Zealand’s sedimentary record will be apparent.
The tons of artificial fertilisers that are now applied to New Zealand farms, along with various metals from industrial pollution, will be visible in the future geochemical record. More ominously, acidification of the oceans will lead to the dissolving of calcium carbonate at increasingly shallower depths. What would have once formed a deep water carbonate sediment may be visible to future geologists as a much thinner bed of clay-rich sediment.
The list of rapid change could go on and on – it certainly seems to me that New Zealand has joined the Anthropocene.
Holdaway, R. N. 1989: New Zealand’s pre-human avifauna and its vulnerability. New Zealand Journal of Ecology 12: 11-25.
McGlone, M. S. & Wilmshurst, J. M. 1999: Dating initial Maori environmental impact in New Zealand. Quaternary International 59: 5-16.
McSaveney, M. J. & Whitehouse, I. E. 1989: Anthropic erosion of mountainland in Canterbury. New Zealand Journal of Ecology 12: 151-163.
McWethy, D. B., Whitlock, C., Wilmshurst, J. M., McGlone, M. S. & Li, X. 2009: Rapid deforestation of South Island, New Zealand, by early Polynesian fires. The Holocene 19: 883-897.
McWethy DB, Whitlock C, Wilmshurst JM, McGlone MS, Fromont M, Li X, Dieffenbacher-Krall A, Hobbs WO, Fritz SC, Cook ER 2010. Rapid landscape transformation in South Island, New Zealand, following initial Polynesian settlement. Proceedings of the National Academy of Sciences of the United States of America 107: 21343-21348