New Zealand's Two Poles: Marlborough and Central Otago

Two regions, one island

Marlborough, in the north-east of the South Island, has three sub-regions: the Wairau Valley, the Southern Valleys and the Awatere Valley. The name is a geographical indication registered in 2018 that covers all three, so a label that says only Marlborough does not tell you which valley the fruit came from.

Central Otago lies far inland in the south of the island, on the 45th parallel south. New Zealand Winegrowers calls it the world's most southerly wine region, but vineyards in Chubut, in Argentine Patagonia, are also planted at about 45° south and make the same claim, so it is one of the most southerly. Central Otago has six sub-regions: Alexandra, Bannockburn, Bendigo, the Cromwell Basin (including Lowburn and Pisa), Gibbston and Wānaka.

The article follows one chain for each region: climate, then rock, then the chemistry of the flagship grape, then the cost of growing it.

Climate: sunny and dry versus continental

Dryness is the main feature of Marlborough's climate (NIWA): most of the region lies in the rain shadow east of the Main Divide, and summer droughts are frequent. It is also sunny: in the Wairau Valley NIWA records more than 2,400 hours of bright sunshine a year, the highest in the region. The Awatere Valley, south of the Wairau, is cooler, drier and windier, often with some elevation.

Central Otago is the country's only wine region with a continental climate: it is dry, with extreme variation between summer and winter temperatures and a marked diurnal range. It lies inland, far from the sea. Each sub-region has its own climate, aspect and altitude: Gibbston, for example, is the highest and coolest, with north-facing slopes, which are the sunward slopes in the southern hemisphere.

The table sets the two regions side by side.

Rock and soil: river gravel and schist

On the Wairau Plains the soil texture changes markedly over short distances, reflecting the braided past of the Wairau River. In one vineyard study, where gravels came to the surface the soil at 30 cm was 1 to 2 °C warmer than on deep silt, vine phenology was more advanced and the fruit was riper on the same date; yield did not differ. The Wairau Valley has old, gravelly riverbed soils and the Southern Valleys heavier soils with more clay.

Much of Central Otago sits on the Otago Schist, a metamorphic rock formed from greywacke and argillite in an accretionary wedge in the Late Jurassic to Early Cretaceous. Central Otago's soils include schist weathered into fine earth, windblown loess from past ice ages and glacial outwash gravels, with alluvial sand, gravel and silt on terraces and valley floors.

There is no evidence that mica or minerals in the rock can be tasted in the wine.

Sauvignon Blanc: where the passion fruit comes from

The passion fruit and grapefruit notes of many Sauvignon Blancs are linked to varietal thiols, mainly 3-mercaptohexan-1-ol (3MH) and its acetate 3MHA. The grape holds no free 3MH; it holds odourless precursors such as the cysteine and glutathione conjugates Cys-3MH and G3MH. During fermentation yeast takes up the precursors, cleaves the carbon-sulfur bond and releases 3MH, then converts part of the 3MH into 3MHA. The conjugates are not the whole story: in one study less than 1% of Cys-3MH became 3MH, and it accounted for only a small share of the 3MH in the wine.

Harvest handling matters. In a study of five Marlborough sites, wines from machine-harvested juice had more varietal thiols than wines from hand-picked grapes at three of the five, and more oxidised juices gave wines with less 3MH and 3MHA. A later New Zealand trial held machine-harvested Sauvignon Blanc at 6, 15 or 24 °C for 2 hours before pressing, to mimic transport: precursor levels differed between 6 and 15 °C, but the finished wines showed no significant difference in 3MH and 3MHA.

So the aroma depends on the vineyard and on every step to the fermenter, and more precursor in the juice does not always mean more thiol in the glass. The figure shows the sequence.

Pinot Noir and the cost of frost in Central Otago

Pinot Noir is Central Otago's flagship variety and covers most of its vineyard area. Frost is a recurring hazard. In November 2012 spring frosts damaged new growth on vines in Central Otago (Otago Daily Times); growers ran overhead sprinklers and wind machines around dawn, and vineyard managers expected some crop loss. It also makes no claim that ultraviolet light, the ozone layer or schist minerals produce a particular flavour.

Exercise: connecting climate, rock and aroma

Question: Using one example from each region, explain how climate and ground can lead to different wine styles.

Model answer: In Marlborough, a dry, sunny valley with gravelly riverbed soils grows Sauvignon Blanc whose passion fruit aroma is linked to thiols released from grape precursors during fermentation and changes with handling at harvest and pressing. In Central Otago, a continental climate with large swings and a frost hazard, on schist-derived soils, grows Pinot Noir whose cost and risk are shaped by that climate. In both regions the claim must stay with what is demonstrated: the link between a specific rock and a flavour is not established.

Mark scheme (6 marks): 2 marks for the Marlborough climate and soil facts; 2 marks for the thiol precursor and yeast release explanation; 1 mark for the Central Otago continental climate and frost; 1 mark for avoiding an unsupported rock-to-flavour claim.