Our climate is changing, rapidly. This, of course, draws several mental pictures: rising global temperatures, ocean acidification, sea level rise, etc. Did you think of sediments running into our oceans? Perhaps you should.
Weathering and sediment runoff are important to look at when researching climate change. They are associated with flooding, they can alter shorelines, and they play a large part in burying carbon in the Earth’s carbon cycle. But how exactly will sediment runoff to the ocean be affected by climate change? One way to look into future climate changes is to look into our past – the last 2.5 million years, to be specific.
Throughout the Quaternary period (2.58 million years ago – present), Earth has been subject to large changes in temperature and precipitation associated with glacial and interglacial states. Many of these cycles operate on timescales of 100s of thousands of years (Figure 1). These changes would have affected biogeochemical cycling and weathering, and thus sediment runoff into the oceans. However, the overall effect of glacial-interglacial climate oscillations on weathering is still debated. Direct evidence regarding global weathering and sediment fluxes over these timescales is scarce.
For answers, researchers have previously looked to elemental isotopes in ancient ocean floor sediments. In numerous studies, isotopes measured in the ocean sedimentary record have recorded glacial-interglacial patterns. Unfortunately, the problem persists that these proxies may reflect changes in the source and the style of weathering, rather than changes in runoff flux itself.
Beryllium isotopes are a different case, and can be used to track relative weathering fluxes. The isotope 10Be is produced when elements in the atmosphere react with high-energy cosmic ray particles; it is then deposited to the oceans and land surfaces. 9Be, on the other hand, is introduced to the oceans by silicate weathering via rivers. Thus, the 10Be/9Be ratio is set by the individual fluxes of the two isotopes into the oceans, and can be used to track weathering fluxes.
Differences in ocean basin mass balances confirm that 10Be/9Be ratios reflect the terrigenous, or land-origin input into the basins. For example, modern seawater 10Be/9Be ratios are lowest in small ocean basins with high terrigenous input, such as the Mediterranean and the northern Atlantic, and are highest in large ocean basins, such as the Pacific.
When Be is incorporated into marine sediments, the 10Be/9Be ratio reflects the overlying water column, providing a record of variations in silicate weathering flux over 100 thousand years to 1 million years. These ratios are preserved in cores of the marine sedimentary record.
In the recently published study investigating 10Be/9Be ratios, researchers studied marine sediment cores (Figure 2) from the Iceland Basin in the Northern Atlantic, among other places. Here, seawater should be most sensitive to inputs of terrigenous glacial debris. The cores (about 172-205 thousand years old) yield 10Be/9Be ratios identical to modern Atlantic seawater, even though they span a glacial-interglacial transition. The study also included cores from other regions of the Atlantic and the Pacific oceans.
By analyzing data from cores and climate models, the study authors found that over the past 2 million years, the terrigenous input into both Atlantic and Pacific basins did not vary much. The uniform nature of 10Be/9Be ratios over timescales of several 100 thousand years provides the first direct evidence that global weathering fluxes over the second half of the Quaternary were constant.
Let’s put that in the simplest terms: It appears the Earth may have a way to balance global variation in silicate weathering during glacial-interglacial cycles, and also for mediating global carbon cycle fluxes.
Although there was little global variability in the weathering, there was regional variability in runoff between glacial and interglacial periods, which actually stabilized the global trend. The study additionally analyzed discharge of 26 major rivers from a climate model ensemble. Model calculations suggest that stability of the global weathering flux was maintained by the spatial distribution of discharge: large changes in high latitudes (which provide a small portion of global runoff), moderate and asymmetric changes in the mid-latitudes, and small changes in the tropics (which provide most of global discharge).
Thus, despite substantial regional variability in river discharge, this research shows the Earth’s ability to balance variations in weathering during glacial-interglacial cycles. Additionally, the absence of cyclic changes in weathering, plus the nature of constant weathering over the past 2 million years, implies an extremely small imbalance in the carbon cycle over this period, even during glacial cycles.
What does this mean for humans? This study focused on extremely long-term climate change fluctuations, which may not even have much pertinence to humans down the road (who knows if we’ll even be around?). It does, however, demonstrate the importance of investigating marine sediments to learn what we can about how our planet reacts to climate change, and what to expect from the carbon cycle.