You peer down your microscope at your sample and see a beautiful diatom frustule.
What kind of environment did this diatom live in? What kind of nutrients does it need to grow? What controlled its abundance - light? nutrient upwelling at the sea-ice edge? the depth of surface mixing? grazing?
What kind of environment existed at the seafloor that preserved these fragile glass shells? The crystals of pyrite and high uranium content of the sediment tell me that organic matter was buried in anoxic conditions. The presence of angular sand grains among the clay reveals that icebergs passed this way.
Certain species dominate when there is a higher supply of iron or silica - Where do these nutrients come from? Are they stable in seawater, or do they constantly need to be replenished? When nutrients are recycled, there is a systematic fractionation of their isotopes - Can we use their isotopic composition to learn something about the water mass in which they lived? Did they leave other clues behind, such as organic molecules that have persisted for thousands of years...?