Ancient phytoplankton serve as important indicators of past water temperatures primarily through the analysis of fossil records and chemical traces. The most relevant and accurate approach among the options provided is:
Fossil records indicate different species of phytoplankton prefer warm water to those that prefer colder water.
Different species of phytoplankton, which are microscopic marine organisms, have specific thermal preferences, meaning certain species thrive in warmer conditions while others flourish in colder waters. By examining the types of phytoplankton fossils found in sediment cores, scientists can infer historical temperature conditions based on the composition and diversity of these species.
Additionally, the chemical composition of phytoplankton, such as isotopic ratios, can also provide clues about the temperature of the water they inhabited. In this way, both fossil identities and biochemical markers contribute to our understanding of historical climate changes and fluctuations in water temperatures over geological timescales.