Disentangling the Palaeoproxy Challenge for the Humboldt Current System and Beyond

The impacts of climate change are being felt by human populations everywhere. The Humboldt Current System (HCS) of the south east Pacific Ocean is one of the most complex and productive upwelling systems in the world, which supports large fisheries on which the people of the region depend. It is heavily influenced by the cycles of El Niño-Southern Oscillation (ENSO) and recent evidence shows that the coastal upwelling dynamics are changing, potentially forced by global warming. This has cascading impacts on the coastal ecosystems, threatening the world's largest fishery, and negatively affecting oceanic and terrestrial biodiversity and the food security and livelihoods of resident populations. Predicting how ENSO patterns will alter the HCS as climate changes, is one of the biggest challenges in climate science today.

To model future climate scenarios, it is important to understand how the regional climate has changed in the past in response to previous global warming. To do so, we use the shells of microscopic marine planktonic organisms called foraminifera (forams). Each foram species lives in a particular habitat and can be identified by its characteristic shell shape. The composition of this shell is a "proxy" for environmental conditions because it reflects the water column conditions (e.g. temperature) in which it was made. After reproduction and death, the shells sink to the seafloor, and accumulate in the sediments generating fossil records dating back millions of years. By taking sediment cores from the seafloor in the HCS, we can use the foram species assemblage and the shell composition "proxy" to reconstruct oceanic and climatic conditions in the past. In this way the foram fossil record represents the foundation stone of palaeoceanography, providing an unparalleled long-term dataset with which to test and improve models for climate change projections.

The use of forams as a palaeoceanographic tool, however, needs to be filtered through a lens of biological understanding. The differing biology of foram species influences shell composition, leading to the routine use of species-specific proxies by palaeoceanographers. However, more recent research has shown that many species have evolved into genetically distinct groups called genotypes, driven by exploitable diverse niches in the water column. We now know that genotypes may look alike and contribute to the same fossil record. Yet, they occupy different niches, interact with different organism and/or are separated seasonally, all of which influence shell composition and lead to a requirement for genotype-specific proxies. Grouped as a single species in the fossil record, these genotypes supply an average temperature for the region, which is useful for understanding past climate over long time scales. However, analysing each genotype independently, or indeed analysing single specimens to understand changes in seasonal patterns through time allows for a much more refined understanding of changing oceanographic and climate patterns. This of course requires knowledge of the genotypes present and their biological preferences, both of which are currently unknown in the HCS, as it is the last remaining globally important oceanographic region to be genetically assessed.

The overarching aim of this pilot project is to complete the global jigsaw and establish the foraminiferal genotypes present in the upwelling and OMZ waters of the HCS. We will then use our developed molecular approach to link these genotypes to their unique biology. We will combine this molecular data with genotype-specific measurements of shell composition to develop genotype-specific proxies. These methods will be directly applicable for research in other ocean regions and will provide palaeoceanographers with the most accurate tools to reconstruct past oceanic conditions, and climate modellers with finely tuned seasonal datasets for ground truthing of climate models.