Think of a mineral chronometer, a stone that tells time. In your mind, it appears nonliving and inert. If you believe, like Martin Heidegger, that humans are uniquely “world-forming,” you might claim the stone is too “worldless” or ontologically deficient to tell time meaningfully. But thinking with inhuman scales of time—opening up to the corporeality of geos—can turn mineral assemblages into powerful temporal agents that alter conceptions of the past, present, and future. Through these alchemical “clocks,” origins and endings intertwine, and the geological and biological blur as life and nonlife “breathe in and breathe out” (Povinelli 2016, 44).

Calcium carbonate (CaCO₃) is a component of many mineral chronometers. Up close, this polymorphic chemical compound takes multiple prismatic forms: trigonal calcite, orthorhombic aragonite, or unstable hexagonal vaterite. Scaled up, it takes on even greater morphological variety: calcium carbonate is the main element of Vietnam’s Sơn Đoòng limestone cave, the chalk cliffs of Dover, Italy’s Carrara marble mine (excavated for Michelangelo's David), and the coral skeletons of Australia’s 2,300-kilometer-long Great Barrier Reef.

That reef hides just below the horizon as I drive toward the world’s largest archive of carbonate coral cores, at the Australian Institute of Marine Science (AIMS). I glimpse the ocean while traveling south through the city of Townsville, passing motels, refineries, dusty quarries, and occasional signs warning of crocodiles. Above, pilots from the nearby army base practice maneuvers in F/A-18 combat jets, tracing arcs through billowing cumulus clouds in preparation for a future war. Out of my window, I see the Pacific Ocean. In that warming and acidifying water, the mineral reef is changing.

Human activity has altered the reef’s future, but how does calcium carbonate affect the temporality of the present? The same matter structuring coral skeletons structures the world around me: It was in the drilling fluids used to extract the fuel combusted by the F/A-18s,
and it is hidden throughout the concrete, steel, and chemical polymers of Townsville and similar built environments. It was also in the toothpaste I used this morning, in my pharmaceutical tablets, and in the paper this essay was drafted on. Millions of tons of this mineral resource are extracted each year to meet global demand, making it difficult to imagine that the calcium carbonate around us was grown. Marine invertebrates like coccolithophores, forams, and coral polyps produce this compound to form protective shells or structuring skeletons. After death, these body parts settle on the seafloor, slowly forming undersea mountains, which have returned, epochs later, as a vital filler, a secret ingredient of modernity (Simonetti 2017).

The road to AIMS ends. Inside the coral core archive, under the watchful eye of climate scientist Janice Lough, I hold a piece of heavy, white calcium carbonate grown by a colony of coral polyps. In my hands, the fragment appears sclerotic. But along with the ten thousand others that fill the cabinets around me, it is a powerful timekeeper. Dr. Lough explains that to become a mineral clock, each fragment was extracted by divers with drilling equipment before being sliced, x-rayed, and photographed by researchers. These fragments now lie in darkened shelves of the archive, but the historical information they hold has traveled far through reports about anthropogenic changes affecting tropical oceans. We know the Anthropocene through calcium carbonate fragments like these, and the scientists that have learned to read them.

Before leaving the archive, I notice a series of images that have been used to do this work: x-rays showing density changes in a coral skeleton. Stuck over one is a length of white tape covered in handwritten marks. The marks measure changes in density through years, translating the coral’s rhythmic carbonate growth into numerical clock time. This ability to translate was learned by accident far from the archive, among the radioactive reefs of Enewetak Atoll. Three scientists (Knutson, Buddemeier, and Smith 1972) went to assess whether the U.S. military’s nuclear tests in the 1940s and 1950s had affected calcareous life in the lagoons. They laid coral slices on light-sensitive paper, and after forty days, strontium-90 hidden in the skeleton unexpectedly materialized as glowing bands in the images, revealing an annual growth pattern that transformed the fragments into “coral chronometers.” We can translate coral time today because humans—the U.S. military—drastically altered the matter inside the reefs of Enewetak Atoll. Coral time and human time melted together in the shadow of a mushroom cloud.

This moment of temporal melt elicits deeper questions. Asking how calcium carbonate structures human time in the Anthropocene leads to coral cores that have been temporally translated to reveal past climatic conditions. Asking about the kind of time calcium carbonate
tells is more difficult, and useful: It helps conceptualize changing configurations of human and more-than-human temporalities in the Anthropocene. Some have described this moment as a temporal “rupture” (Hamilton 2016) or “collision” (Chakrabarty 2018, 5). Neither makes
sense for calcium carbonate, a mineral compound in which temporalities melt together: the moon-linked time of coral polyps, the seasonal oscillations of sea surface temperatures, and the jagged rhythms of technoscience (expressed through radioactive isotopes or synthetic
chemical traces) materially blur and blend together in the calcium carbonate of Earth’s reefs.

To think deeper with temporal melt, I need to travel beyond shallow lagoons to carbonate oozes on the seafloor, where life biogeochemically melts into nonlife. Rachel Carson (1965, 209) describes the Holocene seafloor as “covered with a soft, deep ooze...thick with the skeletal remains of minute sea creatures,” including corals, shells, and other organisms that grow calcium carbonate—a fluid space where years “melt into centuries, and the centuries into ages of geologic time.” Over these long durations, this ooze of once-living bodies “hardens and becomes stone again” (Caillois 1985, 107). Today, traces of human-historical time—radioactive isotopes, toxic chemicals, synthetic polymers, and climate-changed bodies—are inscribed into that calcareous ooze. But into is only one trajectory; the goo of temporal melt opens up other passages.

How could life be thought out of the hardening temporalities that stratify geological time? Asked another way, how could life be held ontologically accountable to nonlife? Among seafloor ooze, I’m thinking of Kathryn Yusoff’s (2016, 19–20) insistence on holding contradictions together to articulate “geologic life as corporeal rather than planetary; as constitutive of subjectivity as well as worlds.” I’m thinking again of a mineral chronometer, a stone that tells time. Imagined as a rock, it appears too inert or sclerotic to be a useful timekeeper. Disintegrating on the seafloor, it becomes something else: a way of tracking the temporal melt of the Anthropocene, for thinking time and life into and out of the ooze.