Written by: Shlomit Sharoni and Itay Halevy

Phosphate is an essential nutrient for all forms of life. Over geologic time scales (millions of years) oceanic phosphate levels drive the productivity of microscopic, unicellular algae named phytoplankton. The organic cellular material of these phytoplankton is the food source on which all the marine biosphere depends. Recently, it was found that marine phosphate levels not only regulate the quantity of the organic matter formed at the base of the food web, but also its quality, i.e., the cellular content of phosphate, which is an essential element for cells (Galbraith & Martiny, 2015; Sharoni & Halevy, 2020; Sharoni & Halevy, 2021). The quality of the organic matter at the base of the marine food web is considered to be a major driver of the productivity and evolution of higher marine trophic levels.

In our study, we focus on the last ~540 million years in Earth's history. This time interval is known as the Phanerozoic Eon, or the eon of “visible life,” which is characterized by the preservation of macroscopic fossils in sedimentary rocks. The increasing diversity, complexity, and “energetics” of the animals preserved in the fossil record over time imply an increasing phosphate availability over the Phanerozoic Eon. However, proxies for marine phosphate availability are underdeveloped, and as a result, so is our understanding of the drivers of the increase in animal complexity.

Mathematical models are often used to fill this knowledge gap. Using information about processes that control the inputs and outputs of marine phosphate, and using geologic proxies to constrain these processes, it is possible place bounds on seawater phosphate levels over Phanerozoic time. Following this approach, we constructed a mathematical model that simulates the evolution of the coupled cycling of phosphate, carbon, oxygen, and alkalinity between the ocean, atmosphere, and Earth reservoirs. We used geochemical and geologic data as model inputs and constrained phosphate levels throughout this important time period.

The main process that delivers phosphate to the ocean is continental weathering. Continental weathering rates depend on climate through a process called the silicate weathering feedback (Walker et al., 1981). According to this feedback, an increase in atmospheric carbon dioxide (CO₂) levels causes an increase in global temperature, precipitation, and runoff, which accelerate continental weathering rates. At a higher rate of weathering, the aqueous products of weathering, silica and alkalinity (e.g., calcium ions) are delivered to the ocean at a higher rate. The alkalinity mass balance then requires an increase in the rate of carbonate mineral burial, which is the main sink of ocean alkalinity. Since carbonate mineral burial removes both alkalinity and carbon from the ocean-atmosphere system, the CO₂-driven increase in weathering rates results in a decrease in the size of the ocean-atmosphere CO₂ reservoir. Hence, this process serves as negative feedback, stabilizing atmospheric CO₂ levels and Earth’s climate.

But not only continental weathering rates depend on climate. The rate of seafloor weathering is also thought to depend on the global average temperature (i.e., climate), and we found that the oceanic phosphate budget critically depends on the relative rates of seafloor vs. continental weathering.

But what is seafloor weathering?

Every million years, approximately 2.7 km² of basaltic oceanic crust is formed at mid-ocean ridges as oceanic tectonic plates spread apart. As the seafloor moves away from this spreading axis, seawater circulates through the upper porous crust, cools the crust and reacts with it. During these reactions, the composition of the circulating seawater changes, importantly, calcium ions are leached from the seafloor basalts. A higher deep-water temperature (driven by warmer climate) accelerates the rate of calcium leaching from the basalts. The calcium ions ultimately precipitate together with calcium to form carbonate minerals, either within the oceanic crust or in the ocean. Either way, the climate-dependence of seafloor weathering removes carbon from the ocean-atmosphere system and stabilizes climate, in a similar way to continental weathering.

Unlike continental weathering, seafloor weathering is not a source of phosphate, and it is even considered to be a minor phosphate sink. So, at times when most of the alkalinity input to the ocean comes from seafloor weathering, the alkalinity is accompanied by less phosphate than if it came from continental weathering. The relative contribution of seafloor and continental weathering to the alkalinity flux is thus an important influence on the phosphate cycle and on seawater phosphate concentrations. By accounting for the climate-sensitivity of both seafloor and continental weathering rates, we found that their relative contribution to the total alkalinity flux was driven by several evolutionary and tectonic events. The first was the evolution and expansion of land plants ~350 million years ago. Prior to the expansion of land plants, which are thought to accelerate continental weathering, the relative contribution of seafloor weathering was high. The second event is the assembly and ultimate breakup of the supercontinent Pangaea. The assembly of Pangaea reduced the delivery of moisture (and rainfall) to the continental interior, leading to a decrease in the relative contribution of continental weathering over the supercontinent’s tenure. As Pangaea broke apart ~200 million years ago, an increase in rainfall over the continents should have increased the relative importance of continental weathering. Together, these two major events caused long-timescale variations in the relative rates of continental vs. seafloor weathering in a way that affected the phosphate budget in the ocean (Figure 1). Specifically, a higher contribution of seafloor weathering to the alkalinity budget in the early Phanerozoic resulted in lower seawater phosphate concentrations. We suggest that the evolution of land plants and then the breakup of Pangaea increased the role of continental weathering, the influx of phosphate to the ocean, and the concentration of phosphate in seawater.

Overall, our study highlights the ways in which evolutionary and tectonic events changed the balance between seafloor and continental weathering, how this affected the phosphate budget, and in turn, the productivity of marine ecosystems and even the evolution of life in the oceans.

Read the full article, now out in Nature Geoscience.

References:

• Galbraith, E. D., and Martiny A. C., (2015) A simple nutrient-dependence mechanism for predicting the stoichiometry of marine ecosystems. Proceedings of the National Academy of Sciences 112.27: 8199-8204
• Sharoni S. and Halevy I., (2020) Nutrient ratios in marine particulate organic matter are predicted by the population structure of well-adapted phytoplankton. Science advances 6.29: eaaw9371
•  Sharoni S. and Halevy I., (2021) Geologic controls on phytoplankton elemental composition. Proceedings of the National Academy of Sciences .119 (1) e2113263118. 
•  Walker, J. C. G., Hays P. B, and Kasting J. F., (1981) A negative feedback mechanism for the long‐term stabilization of Earth's surface temperature. Journal of Geophysical Research: Oceans 86.C10 (1981): 9776-9782.‏