We measured the three main types of plankton that produce calcium carbonate in the ocean, at the same time and location. While coccolithophores were the biggest contributors, we found that planktonic gastropods, not foraminifera, were the second largest contributor. This challenges the current view and improves our understanding of how these organisms influence oceans’ carbon cycling.

We examined ocean sediments from the tropical Indian Ocean to study water column structure and carbon cycling during the mid-Piacenzian Warm Period, about 3 million years ago, when atmospheric carbon dioxide levels were similar to today. Our findings reveal persistent upper ocean stratification and niche separation among plankton groups, which limited nutrient mixing and carbon export to the deep ocean. These results highlight how ocean layering can influence climate feedback in a warmer world.

We present new high-resolution data of the relative contribution of the two main pelagic carbonate producers (coccoliths and foraminifera) to the total pelagic carbonate production from the tropical Atlantic in past warm periods since the Miocene. Our findings suggests that the two groups responded differently to orbital forcing and oceanic changes in tropical ocean, but their proportion changes did not drive the changes in overall pelagic carbonate deposition.

At present, under low eccentricity, the tropical ocean experiences a limited seasonality. Based on eight climate simulations of sea surface temperature and primary production, we show that, during high-eccentricity times, significant seasons existed in the tropics due to annual changes in the Earth–Sun distance. Those tropical seasons are slowly shifting in the calendar year to be distinct from classical seasons. Their past dynamics should have influenced phenomena like ENSO and monsoons.

Coccolithophores contribute to the global carbon cycle and their calcite structures (coccoliths) are used as a palaeoproxy to understand past oceanographic conditions. Here, we compared three frequently used methods to estimate coccolith mass from the model species Emiliania huxleyi and the results allow for a high level of comparability between the methods, facilitating future comparisons and consolidation of mass changes observed from ecophysiological and biogeochemical studies.