Controls on the chemical composition of marine organic matter today and over Earth history

The chemical composition of planktonic organic matter, expressed as the C:N:P ratio, dictates the flow of matter and energy through the marine food web and plays a crucial role in the biogeochemical carbon, oxygen, phosphorus and nitrogen cycles. Using a combination of field and laboratory data, we tested the major explanations for variation in the C:N:P of marine organic matter (around the global average "Redfield ratio" of 106:16:1). We found that variations in the taxonomic composition of the phytoplankton community best explains these C:N:P variations, with a more minor role for acclimation to local nutrient levels. With these insights and a model of the geologic history of the phosphorus cycle and climate, we suggest that the C:P and N:P ratios of marine organic matter have decreased from high Paleozoic (541 to 252 million years before present) values of ~120-250 and ~15-40, respectively, towards their modern values. This decrease was driven mostly by an increase in "continental weatherability"—the susceptibility of the continents to chemical weathering. The weatherability increased in response to two main events: (i) the expansion of land plants about 400 million years ago, and (ii) the breakup of the supercontinent Pangaea about 240 million years ago. The resulting changes in the influx of phosphate to the oceans drove a decrease in marine organic matter C:P and N:P, and may have had a role in the succession of the major phytoplankton groups observed in the marine fossil record.