TURBINE | KAPOHAU

REBECCA PRIESTLEY

 

Warning: sea levels can rise quickly

From Ice Lens: 15 million years in Antarctica

 

One of the best places in the world to see evidence of past sea level changes—marking sequential warming and cooling of the Earth’s climate—is in Whanganui, just a couple of hours’ drive north of Wellington. I was keen to see what this evidence looked like, so one spring morning I started my day by meeting a couple of geology colleagues at the university café that’s just down the stairs from my office. Cliff Atkins was there when I arrived, wearing jeans, boots and a heavy shirt, buying himself an Americano and a chocolate Afghan to go.

‘Breakfast of champions,’ he said with a grin, as I ordered my trim flat white.

Tim Naish turned up next. In a houndstooth jacket and glasses he can look quite professorial—he is a professor, after all—but on this day he was wearing jeans, a North Face fleece jacket, and mud-covered tramping boots. He carried a pile of books, including Charles Fleming’s The Geology of Wanganui Subdivision—a landmark 1953 study of what we now call the Whanganui Region—a garden spade, and a backpack.

In the car park we loaded everything into one of the geology school vans, and hit the road, chatting about geology field trips from the past, sea level rise, and geologist Charles Fleming. Tim never got to meet Fleming—he died in 1987, when Tim was a student but before he started working in Whanganui—but he’s had a huge influence on Tim’s work.

‘What Charles did was good old-fashioned stratigraphy. He worked out past water depth changes from the sediments and fossils using old school palaeontology and sedimentology and recognised about 40 cycles of sea level change in what he called the Pleistocene.’ The Pleistocene—known colloquially as ‘the Ice Age’—was an epoch from about 2.5 million years ago to 11,700 years ago, during which northern hemisphere rocks and landscapes showed evidence of only four glaciations and four interglacial warm periods. ‘So Charles was scratching his head and saying, ‘what’s going on?’

In the decades that followed, other scholars found evidence for many more glacial and interglacial periods on Earth over this time, rather than the four glaciations previously recognised.

In Whanganui, we now know the rocks catalogue a changing global sea level related to Antarctic and northern hemisphere ice melt and freeze. Put simply, when global temperature increases, the ice melts, and sea level rises. When temperatures drop, the glaciers and ice caps grow, and sea level falls. Fleming looked at the exposed rocks in cliffs and outcrops, but Tim and others have drilled deep into the farmland, to get a continuous record of sediment deposition—sandy fossil-rich sediments from when sea level was starting to rise, deep-water muds from when sea level was higher—over the past five million years.

‘Whanganui, that’s where I cut my teeth as a geologist,’ said Tim as we drove north. Tim started working here in the early 1990s. ‘We were starting to form this picture that there must have been a reasonably dynamic Antarctic ice sheet contributing to global sea level changes.’ Geological evidence from the Transantarctic Mountains suggested the East Antarctic Ice Sheet, which covers most of Antarctica, was dynamic, and had halved in size about three million years ago during the Pliocene warm period.  Geomorphologists disagreed, insisting the ice sheet had remained stable and intact over this time period. ‘So there was this debate raging between the stabilists and the dynamicists.’

It was difficult time, said Tim. ‘People had polarised themselves and it had to be a stable ice block that did nothing, or it was this wildly dynamic ice sheet coming and going.’

Whanganui provided evidence that helped scientists to realise that the truth was somewhere in the middle. George Denton, one of the stabilists, visited from the US in 1998, 14 years after his 1984 paper affirmed his position that the East Antarctic Ice Sheet had been intact and stable for the past 14 million years, and conceded there was evidence for about 20 metres of sea level change, ‘but not half the Antarctic ice sheet,’ said Tim. Another piece of the puzzle came from the Andrill project, the international effort in which deep marine sediment cores were drilled from two locations near Antarctica’s Ross Island, to get a picture of changing temperatures and ice extent over the past 20 million years. Tim was co-chief scientist on one of the two drilling projects.

‘When we drilled Andrill, we understood that the West Antarctic Ice Sheet had indeed collapsed and grown. But the West Antarctic Ice Sheet—at least the marine portion of the sheet—is only capable of 3-4 metres of sea level change.’ More recent drilling projects have revealed that the margins of the East Antarctic Ice Sheet have been prone to melting over the past 35 million years and have also contributed to sea level rise.

South of Whanganui city we left the highway, then pulled over on the grassy side of a winding hill road. Below us, down a tree-covered bank, was flat green farmland, divided by long fences, and dotted with sheep, an old brown shed, and a few tall trees. The floodplain. Beyond that, a wide, flat, grey-blue river meandered around a scrub-covered gravel island. But the real view was on the other side of the river. Immediately above the water was a sharp cliff, 100 metres or so high. From this distance, the cliff was shades of white, grey and yellow, with a few scrubby plants poking out from wherever they could get a hold. In some places there was a sharp horizontal boundary between the colours, clearly showing a sandy yellow deposit—usually indicative of a near shore environment—on top of a light grey muddy deposit typical of deeper water.

We stood gazing at the bluffs on the east side of the Rangitikei River. ‘There it is, New Zealand’s version of the Grand Canyon,’ said Tim. It was kind of a joke, this was nothing compared to the Grand Canyon, just some high cliffs. But what makes a canyon or gorge like this exciting to geologists, is that the exposed sediments show layers and layers of past history that might otherwise only be revealed by a costly drilling project. The sediments are hard to get to, but Tim has had a close look, abseiling down the cliff, which he describes as ‘a couple of rope lengths high,’ to take photographs and samples.

Tim mapped this area for his PhD, and knows it well. Our next stop, on the east bank of the river, is one of his favourite sites. We got there by climbing over a few fences and walking through grassy fields peppered with cowpats. Tim had been here many times before—he often brings students or visitors here—and he knows the farmer. We were here to see an outcrop of the Hautawa shell bed, one of many fossil-rich layers of sediments in the Whanganui Basin. Airini Beautrais wrote about one of the other key shell beds, the Wilkes, in her poem Pandora’s Box, inspired by a newspaper interview Tim did back in 2012. The Wilkes shell bed is spectacular, says Tim. ‘Really thick—15 metres—and completely dominated by these bloody big, giant oysters, Crassostrea.’

 

When you come to this cliff,

                                                       cleared and cut-back,

roughly risen,

                           you read in the rock

strangely pertinent

                                        Pliocene prophecies

silent for centuries,

                                       scripted in shells.

wrote Beautrais. The Hautawa shell bed is only half a metre thick, but it’s the most important shell bed in the Whanganui sequence, said Tim.

It was clear when we reached our destination. We were 40 kilometres from the current shoreline but we were standing on a flat rocky platform of yellow rock crammed with shells. I squatted down low to look closer. I could see white crescents of scallop shells, which Cliff told me were a cold water species now found only in the Otago region of the South Island. There were some narrow, pointy-spired gastropods.

Cliff and Tim used their rock hammers to dig some shells out of the soft young sediments and passed them to me as they identified the species.

‘Zygochlamys delicatula,’ Cliff said of the scallops.

‘A nice brachiopod as well.’

‘And these are . . . purupucardia . . . yeah, that’s the one.’

Tim said there were even cold water crab fossils and some penguin bones in the shell bed—all evidence of a colder climate when sea level was lower. At the bottom of the bed is the first appearance, at this location, of the cold water scallop, which Tim says marks the beginning of the Pleistocene, the ‘ice age’ that began 2.5 million years ago. ‘This was the beginning of the big global sea level fluctuations and a cold climate in New Zealand. But as you go up the shell bed the Zygochlamys disappear and you go into the interglacial warm period where there’s deeper water and warmer species.’ This sequence represents 20,000 years of sediment deposition, but it was an eventful 20,000 years. ‘We go from cold water climate to warm water in half a metre of shell bed.’

‘Calcareous things,’ Cliff said to sum up, and we stood to look at the cliffs behind us, where the sedimentary sequences that sit above the Hautawa shell bed showed layers of deposits, shallow water sands and deeper water silts. They represent another 20,000 years, from a time when sea level was falling and lots of sediments were washing into the region from rivers at the top of the South Island.

Why Whanganui? I asked Tim.

‘The key thing about the Whanganui Basin is it’s subsiding, it’s been sinking for 5 million years. And what that’s done, is create a big hole in the ground. A 5-kilometre-deep hole. As the hole’s being created, it’s continuously filled up by sediments.’ For millions of years, this whole area was underwater, part of the near shore continental shelf. ‘And that’s the absolutely sweet spot if you want to capture sea level changes of 0 to 100 metres. So it’s this perfect balance between subsidence and sediment supply that maintains the water depths that are very sensitive to these changes in global sea level.’

There are other sedimentary basins like this, but most of them remain underwater. ‘That’s why Whanganui is so unusual,’ said Cliff, who brings his students here on field trips. ‘Over the last few hundred thousand years the eastern margin of the Whanganui Basin has been tectonically uplifted, exposing these geologically very young rocks.’

Next, we drove to the coast, a small settlement south of Whanganui called Kai Iwi Beach. At the top of some steep steps down to the sea was a red and white sign. ‘Warning: sea levels can rise quickly,’ it said. It was warning walkers that at high tide the ocean speeds in to touch the crumbling cliffs, leaving no beach behind, but it got us talking about the sea level changes these rocks are revealing.

Down the steps, across a small river, was the beach. From our position on the wet sand, between the yellow and grey layered cliffs and the waves, Tim imagined what would be happening right here if sea level changed. ‘If you drop sea level by 50 metres you’d have to walk 10 kilometres west to get to the shoreline. But if sea level rose 50 metres, then all of a sudden you’d be on the inner shelf under 50 metres of seawater.’ These sorts of changes in sea level are recorded in the cliffs beside us, and in the cliffs we saw earlier in the day, and in the fossil bed by the Rangitikei River.

‘You see very well-sorted sandy sediments with fossils in them that you’d find on the beach today, then all of a sudden you go up several tens of metres and you’re in these blue-grey mudstones that represent 50-100 metres water depth.’ These regular cycles, between beach sand and continental shelf mud, show a record of 50 different sea level changes over the last 2.5 million years.

By subtracting the known land subsidence rate, geologists can work out the amount of sea level change over time and use the fossils and volcanic ashes to date when the changes occurred.

We sat on some sandstone boulders on the dark wet sand. As waves chimed in from the sea, Cliff and Tim chatted about what the rocks can tell us about climate change. Projections published by the Intergovernmental Panel on Climate Change, which represents the international scientific consensus, show temperature increase by 2100 as being between 1.5°C and 4°C, said Tim, depending on whether we manage to reduce our greenhouse gas emissions.

The planet is warming because of the greenhouse gases we’ve been releasing into the atmosphere since the beginning of the Industrial Revolution. If we can get it together as a planet to sufficiently reduce our CO₂ emissions, we can reduce the temperature we ultimately end up with, the Earth’s new normal. ‘Ideally, we’d like to keep Earth’s surface temperature increase below 2°C. That’s considered the threshold for the more dangerous consequences of climate change.’

We were on the beach in mid 2015, before the Paris Agreement—in which the world agreed to take steps to limit global warming to 1.5-2°C by 2100— was signed. It was clear the world was not doing nearly enough. ‘The path we’re on is at the upper end of that. We’re tracking towards 4°C of warming by the end of the century,’ said Tim.

‘The last time the world was 4°C warmer, the Ross Ice Shelf was gone, the West Antarctic Ice Sheet was gone. Sea level was about 20 metres higher than it is today.’ He was talking about the mid-Miocene climate optimum, about 15 million years ago. If sea level was that high, the boulders we were sitting on would be under 20 metres of water and the shoreline would be many kilometres inland.

‘We know the end game. We know that if we keep CO₂ levels of 400 parts per million or more in the atmosphere for long enough, that’s where we’re heading.’

Why, though, are some people still arguing about this, even denying that climate change is happening? Tim doesn’t know, but he’s seen a change in the public mood in the two decades he’s been working on climate science.

‘Ten years ago, people were fairly sceptical, perhaps because they didn’t understand the science. I think with the last two IPCC reports both reinforcing the science very clearly, that the majority of the public now accept that the world is warming because of fossil fuel use and greenhouse gas emissions. What I would say now is why are we not doing anything about it? That’s the key question. What are we going to do?’

By now, the tide was coming in and we needed to get off the beach.

At the end of the day we drove home along roads that ran through bright green fields, the setting sun bathing the dairy cows in a golden light.

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