Using sedimentation rates to infer long-term global climate change
Over geologic time scales, the Earth naturally captures carbon dioxide from the atmosphere through weathering of silicate rocks and sequesters it via the production of carbonate rocks. Ultimately, subduction can return these rocks to the Earth’s interior and carbon dioxide is once again emitted into the atmosphere by volcanism. Thus, understanding the history of erosion and, by association, weathering of our planet’s surface will help us understand this important geochemical cycle and its relationship to climate.
Paleoclimate records for the past ~50 million years show overall global cooling with higher-frequency fluctuations (see plot of δ18O measurements, a proxy for temperature, at left). The annotation on the right side of the figure denotes when researchers think continental glaciation was initiated — approximately 30 million years ago for Antarctica and 5-8 million years ago for the northern hemisphere.
What caused this long-term global cooling?
Some researchers have hypothesized that increased tectonic uplift (especially the Himalayas) and the increased weathering that came with it is one of the primary causes of this long-term global cooling*. That is, as more carbon dioxide was withdrawn from the atmosphere and naturally sequestered, the Earth cooled. One line of evidence geologists have used to infer changes in rates of erosion and weathering is the depositional record.
Sediment-dispersal systems transfer eroded material from mountainous uplands to sites of sediment accumulation in alluvial lowlands, coastal deltas, the continental shelf, and the deep sea. Because erosion in one location is balanced by deposition in another location, some geologists are using changes in deposition rates to infer changes in erosion rates. This seems like an elegant solution but, as usual, it’s not quite so simple.
I was delighted to see this paper by Willenbring and von Blanckenburg about late Cenozoic erosion/weathering rates come out in Nature last month. There’s a lot to say about this paper and its implications, too much to cover in this one blog post. I recommend this great review article by Yves Godderis, which highlights the global paleoclimate implications of the study nicely. Here, I will focus on one aspect I find fascinating: how erosion and deposition is recorded in the stratigraphic record.
Willenbring and von Blanckenburg revisit an inherent measurement bias demonstrated by Peter Sadler in his 1981 seminal paper “Sediment accumulation rates and the completeness of stratigraphic sections.” I’ve written about the ‘Sadler Effect’ before on this blog (read this post from 2007) but to make a long story short — the longer the measured time interval the smaller the sedimentation rates and vice versa.
This phenomenon arises because the accumulation of sediment is unsteady — that is, at one location it varies over time. As longer durations are measured in a succession, more hiatuses, or periods of no deposition, are captured and, thus, the rate (thickness/time) is lower. Conversely, rates measured from very short durations can be quite high. Think about it — if you go out to a river mouth when it’s flooding and measure the thickness of sediment that piled up over a day and converted it to a yearly rate by simply multiplying by 365 you’d get a huge rate that wildly overestimates the actual yearly rate (unless, of course, that river is flooding every single day of the year). This is intuitive and few would attempt to make such a conversion, but this is essentially what is happening when comparing sedimentation rates that were measured from different time scales. Similarly, geomorphologists have discussed the unsteadiness of processes such as erosion and uplift^.
Willenbring and von Blanckenburg show this by plotting various process rates against time. The plots at right show, from top to bottom, (a) ocean-basin sediment accumulation rates, (b) volumetric erosion rates, (c) sediment accumulation rates in Asian offshore basins, and (d) global denudation rates. Note how all of them show a huge uptick in rates during younger time intervals. The point here is that this apparent increase in rates is an artifact of measuring unsteady processes — it isn’t real. The inset log-log plots of the same data show the Sadler Effect clear as day.
So, does this mean we can’t use measured rates of deposition to say something about the variability of processes in Earth history? I think we can but we need to significantly increase our understanding of rates of processes at multiple time scales. Careful documentation of sediment-dispersal systems with ever-improving geochronological tools is one approach to unraveling these complex temporal relationships. Numerical/physical experimental methodologies can be designed to address process rate questions as well. Ultimately, the integration of multiple approaches will lead to a better understanding of the complex temporal relationships of sediment erosion, transfer, and deposition and how we can utilize the stratigraphic record to reconstruct Earth history.
Willenbring, J., & von Blanckenburg, F. (2010). Long-term stability of global erosion rates and weathering during late-Cenozoic cooling Nature, 465 (7295), 211-214 DOI: 10.1038/nature09044
* just one example is this Raymo and Ruddiman paper from 1992
^ Gardner et al. (1987); Geology