Records of Holocene Southern Hemisphere westerly wind variability in Fiordland, New Zealand

Abstract

The mid-latitude westerly winds are a strong, zonally symmetric atmospheric circulation feature in the Southern Hemisphere that directly influence climate and carbon cycling in the middle to high latitudes. In order to forecast potential future changes in the Southern Hemisphere westerly winds (SHWW), which would in turn affect regional and global climate, a better understanding of the past variability of the circulation system and associated climate responses is critical.

Fiordland, New Zealand (44.5-46.5 degrees S) is a promising but understudied site for reconstructing SHWW variability, located at the northern edge of the modern wind field maximum. Fourteen E-W-oriented fjords sit windward of the Southern Alps mountain range, which intercepts the incoming SHWW. The resulting extreme orographic precipitation (>6 m/yr) delivers large amounts of terrestrial material to semi-restricted, rapidly accumulating fjord basins. Due to the strong correspondence between SHWW strength and regional precipitation, proxies that reconstruct past precipitation changes should effectively capture SHWW variability. The SHWW also drive the estuarine circulation patterns in the fjords, thus proxies for circulation changes will also provide information on past SHWW strength at this latitude.

While a suite of proxies related to precipitation patterns and water column circulation exist, many are influenced by a multitude of factors. Thus, each system must be calibrated in a modern setting to test our conceptual understanding of these tools. I present a thorough modern investigation of proxies for precipitation (concentrations and isotopes of carbon and nitrogen and lipid biomarker distributions), which display predictable behavior in modern fjord sediments. Namely, more terrestrial material is located near fjord heads, and more marine material accumulates near fjord mouths. Additionally, restricted inner basins that are intermittently anoxic are enriched in redox-sensitive trace metals, providing a useful tool for reconstructing past water mass restriction. I demonstrate that the long-term primary control on changes in precipitation and circulation proxies is likely to be shifts in the SHWW rather than other local factors.

I further investigate modern circulation processes through radiocarbon cycling in the fjords. The modern marine radiocarbon reservoir age, reconstructed using paired terrestrial and marine macrofossils, is 59 +/- 35 years older than the global modeled reservoir age. In contrast, four reservoir age estimates from throughout the Holocene are younger than the global modeled average. This result suggests that the westerly influence on circulation and delivery of radiocarbon-enriched (i.e., younger) terrestrial material was stronger during the early Holocene, and has potentially decreased in the past millennium.

I also demonstrate the unclear signal of the emerging uranium "stable" isotope redox proxy in fjord waters and sediments. Surprisingly, no redox-driven isotopic fractionation is measured in an anoxic water column, and two euxinic basins show fractionation in opposing directions. This result calls into question the assumed fractionation factor between seawater and sediments in anoxic settings, which has been applied in global reconstructions of past ocean oxygenation.

Using the modern process studies to guide interpretation and further develop conceptual models, a suite of carefully selected proxies is applied to a sediment core recovered from Deep Cove. The basin has the highest measured carbon accumulation rates in all of Fiordland and provides an unprecedented opportunity to generate a highly resolved record of SHWW change in southwest New Zealand over the last 1,600 years. I find several sub-centennial scale intervals of regional SHWW strength fluctuations. I show that the Deep Cove record agrees well with an existing SHWW reconstruction from Fiordland, but conflicts with others from across the Pacific basin. This comparison highlights the regional variability of the SHWW belt, which, until recently, has been largely overlooked. Further constraints on the asymmetries in wind strength and position are crucial to better forecast the linkages between the SHWW and regional and global climate.