Unlocking The Rocks

.

CONTRIBUTION TO THE 2008 GREENOCK CREEK VINEYARDS AND CELLARS NEWSLETTER

by PHILIP WHITE – September 2008

Time for some rough science. While global warming is such a hot topic cough cough it seemed perfectly appropriate to take a little geology lesson: geology shows we’ve had global warming before. So, like, how bad can things get?

Before you check out Snowball Earth on Wikipedia, let me quote a report of Hoffman, Kaufman, Halverson and Schrag, suggesting one of the things that happened at the bottom of the Neoproterozoic groups which underly Greenock.

“… biological productivity in the surface ocean collapsed for millions of years. This collapse can be explained by a global glaciation (that is, a snowball Earth), which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern level….resulting in a warming of the snowball Earth to extreme greenhouse conditions. The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing the cap carbonate rocks observed globally.”

I’m sure they make a big difference, but there were no Hummers in those days.

It was also high time the Barossa seriously compared local wines according to their geological sites. But fearing that they may end up with a geology somehow less desirable than others, some vignerons have opposed such an approach for decades. Their excuse? They say they don’t want an appellation imposed like those of France. My response? It’s not a man-made imposition. It’s in the ground beneath you. It was there first.

So a highlight of my thirty years of wine writing finally exploded like a firework, when, in June, I was invited to assist the Barossa winemakers assemble a blind tasting of 52 unfinished 2008 shiraz wines from across the breadth and length of the Valley, from Lyndoch to Kalimna. These were tasted in brackets roughly according to their geological sources, as set out in The Geology of the Barossa Valley, a brochure and map by revered government geologist, W. A. Fairburn. This work, which has the authority of having been gnawed over by the author's scientific peers, is available from Primary Industry and Resources SA. We also had input from the contrary geologist-turned tea-trader turned wine-merchant turned wine-blogger David Farmer, who is writing a book on Barossa geology, and who disagrees with some of Fairburn's mapping.

The tasting was astonishing, while predictable enough. Neighbouring vineyards in each precinct offered flavours and aromas in common, and these characteristics changed from precinct to precinct. This pioneering tasting, conducted with thirty wine writers from around the world, will no doubt be the first of many such exercises, and marks the beginning of a whole new database of gastro-geology.

The base rocks around Seppeltsfield, the Greenock Creek homestead, and Roennfeldt Road are all from that Neoproterozoic, the geological era in which multi-cellular life first appeared. This era stretches from about 550 million years ago to 1.2 billion years. Just for reference, the Universe seems about 13 - 15 billion years old; Earth about 4.5 billion. While these old rocks are generally below the topsoil, they do extrude, and have of course influenced and added to the formation of much of that soil, which very directly influences the flavours of the grape.

But it’s those base rocks that really interest me, particularly when I read back labels and brochures claiming “our vines are grown in some of the oldest soils on Earth”. Most of the Barossa geology formed in the Tertiary and Quaternary, the last 50 million years; its soils are only tens of thousands of years old: most of them are such recent alluviums they’re barely soils at all. “To the geologist, soil is the dandruff of the Earth”, my friend Wolfgang Preiss, Chief Geologist of the Geological Survey in PIRSA, sagely uttered on a recent field trip.

The Greenock Creek vineyards are on four quite distinct formations. The creeklines, both at the homestead and Roennfeldt’s, are very recent alluviums, just tens of thousands of years old. The cabernet, the Creek Block shiraz, and most of the Apricot Block are in such alluviums. These deposits fill the creeklines between the sharply-dipping older strata which protrude in the ridges.

These include the blue-grey dolomitic siltstones - Willunga slate, for example - of the Tapley Hill Formation, deposited as sediments in still deep lakes that once covered the area about 750 million years ago. The Seven Acre and part of the home blocks are in this formation.

Below that lies the Yudnamutana Subgroup. This dark mix of siltstone-derived soil with blotches of bright quartzite and pebbly dolomite is up to 800 million years of age. These layers reappear in Clare and the Adnyamathanha country of the North Flinders. They are pocked with dropstones, which were deposited by floating glacial ice floes. These rocks were one of the fascinations of the great geologist and explorer, Sir Douglas Mawson. Alice’s and part of the Apricot Block are in Yudnamutana.

The Hopeless Hill, on Roennfeldt’s, is on the border of the Yudnamutana and the underlying Burra Group, where we get to really ancient glittery micaceous schists, metasiltstones, calcsilicates and quartzites. These are as old as it gets in the Barossa. The Roennfeldt shiraz, cabernet and the Cornerstone Grenache are in Upper Burra.

In geology, there are many arguments. But having finally got this sorted better than ever before, I’ll never approach Greenock Creek wines in the same way. The distinguishing characters of each vineyard already make much more sense, and the differences between the Greenock Creek/Marananga/Seppeltsfield/Roennfeldt vineyards and the much younger formations in the rest of the Valley become even more meaningful.

So that’s the ancient history. Contemporary history includes the salination, through introduced irrigation water, of the young creekline sediments and clays. And, of course, it includes current weather and climate. People are finally beginning to understand my salination theories. Now, the pace at which the climate is changing must force closer investigation, much quicker than anybody has imagined necessary. If, in a couple of decades, man can change the soil sufficiently to kill a vineyard, like the poor old Creek Block, never irrigated, but dying through salination from upstream irrigators, we can surely bugger up our air.

Or maybe old Mother Earth will just carry on doing what she did before. Now and again, as geology shows, something makes her lose her cool.

PS.

Just to put all this perspective, Don Francis, professor of geology at McGill University in Montreal, has since reported in Science journal that his team has found a sample of Nuvvuagittuq greenstone on Hudson Bay that they believe is 250 million years older than any other rocks known.

"The rocks contain a very special chemical signature - one that can only be found in rocks which are very, very old," he said. "Originally, we thought they were maybe 3.8 billion years old. Now we have pushed the Earth's crust back by hundreds of millions of years. That's why everyone is so excited."

Before this study, the oldest whole rocks were from a 4.03 billion-year-old body known as the Acasta Gneiss, in Canada's Northwest Territories, and the oldest Australia had to offer were 4.36 billion years old mineral grains called zircons from Western Australia.

The greenstone contains fine ribbon-like bands of alternating magnetite and quartz, typical of rock precipitated in deep sea hydrothermal vents - which have been touted as potential habitats for early life on Earth.

"These ribbons could imply that 4.3 billion years ago, Earth had an ocean, with hydrothermal circulation," said Francis. "Now, some people believe that to make precipitation work, you also need bacteria. If that were true, then this would be the oldest evidence of life. But if I were to say that, people would yell and scream and say that there is no hard evidence."

(This additional information was taken from http://news.bbc.co.uk/2/hi/science/nature/7639024.stm )

.