Lawrence, K.T., D.M. Sigman, T.D. Herbert, C.A. Riihimaki, C.T. Bolton, A. Martinez-Garcia, A. Rosell-Mele, and G.H. Haug (2013), Time-transgressive North Atlantic productivity changes upon Northern Hemisphere glaciation, Paleoceanography, 28, doi: 10.1002/2013PA002546
Approximately 2.7 million years ago (2.7 Ma), the Earth’s climate transitioned to much cooler conditions, such that large glacial ice sheets grew and persisted in the Northern hemisphere. The intensification of northern hemisphere glaciation marks the change from the Pliocene to Pleistocene epoch (geologic time subdivision). Approaching this 2.7 Ma boundary, changing climatic conditions had already been underway. Before discussing the observations and findings of this study, it is important to understand how climate scientists determine what the Earth’s climate system was like in the distant past.
Climate scientists often rely on proxy datasets to determine what the climate was like during the past. A proxy is something that unknowingly records climatic conditions at the time of its existence that can be measured today. For example, foraminifera (small marine protists) grow a shell made of calcium carbonate. The materials for the shell are taken up from seawater. When these critters die, their shells rain down to the seafloor where they form sediment. Scientists can collect the sediment and measure isotopic ratios of the shell, which reveal information about the ocean’s chemistry at the time the shell was made.
In this study, scientists used a multi-proxy approach. Just as a financial advisor would recommend a diversified portfolio of investments to reduce risk, climate scientists look for agreement amongst multiple proxies to have a more robust record of climate in the past. The authors used the mass accumulation rate of biogenic opal and alkenones as proxies for productivity. Both opal (a form of silica) and alkenones (long chain organic compounds) are created by phytoplankton in the ocean, and rain down to the seafloor after the organisms die. By determining how much opal or alkenones were accumulating in the sediment record at a given time, scientists can infer the relative amounts of phytoplankton living in surface waters at this given time.
Phytoplankton are primary producers of the ocean, using sunlight and carbon dioxide to create food, similar to trees on land. The mass accumulation rate of opal and alkenones, which are produced by these phytoplankton serve as a proxy for productivity, or the rate of the generation of biomass. Phytoplankton rely on nutrients (e.g. nitrate, phosphate), which sustain productivity. Nutrients are typically used up in warm surface waters and are supplied by cool, deeper waters. Cool deeper water would never make it to the surface to drive productivity without ocean circulation. It is important to realize that high productivity as recorded by proxies suggests ocean circulation favorable for supplying nutrients to the surface waters where phytoplankton live. This favorable circulation is called upwelling.
Upwelling, the westerlies and productivity:
Upwelling is largely driven by wind. The westerlies are a prominent zonal (following lines of latitude) band of winds blowing from west to east over the mid latitudes in both the northern and southern hemispheres. Blowing across the ocean, the westerlies impart a friction force on the surface of the ocean, although the net movement of the surface waters is perpendicular to the direction that the wind is blowing. This perpendicular movement arises due to the Coriolis force, an apparent force due to the rotation of the Earth. In the northern hemisphere, the westerlies and Coriolis force the water south of the wind band, a phenomena known as Ekman divergence. As the surface water is moved away from the westerlies, deeper, nutrient-rich water rises to the surface to take its place. This upwelling caused by the westerlies leads to high productivity proximal to the westerly wind belt.
Now that we have an understanding of proxies and the connection between nutrients, productivity and upwelling, let’s have a look at some of the observations from this study. The study utilized Ocean Drilling Project (ODP) sediment cores from polar, sub-polar, mid and low latitude sites from the North Atlantic, South Atlantic and Southern Oceans. The authors used these sediment cores to determine the change in productivity, based off of the mass accumulation rates of biogenic opal and alkenones. Figure 1 shows these proxy records at each site.
One notable observation is that the polar and sub-polar sites (Figure 1: a, d, e, j) show large decreases in productivity through time, evidenced by decreases in the mass accumulation rates of opal and alkenones. Mid and low latitude sites (Figure 1: b, c, f, g, h) show just the opposite; increases in productivity through time. A more detailed look at the timing of the productivity “crashes” at the polar and sub-polar sites reveals that the most polar sites experienced crashes around 3.3 Ma (Figure 1: d), whereas sub-polar sites saw productivity crashes closer to 2.7 – 2.5 Ma (Figure 1: a, d, j). The timing of productivity increase in mid to low latitude sites typically occur around 2.7 Ma (Figure 1: b,c, f, g, i), with the exception being ODP site 1090 (Figure 1: h) increasing around 1.3 Ma. ODP site 1090 had been found to be iron limited by previous studies, which is implicated in the delayed productivity increase.
As was discussed earlier, the quantity of nutrients being upwelled to the surface largely controls productivity. In this study, the authors suggest the changes of productivity are controlled by location of the westerly wind belt, which drive upwelling. Throughout the Pliocene (> 2.7 Ma), the westerlies were located at higher latitudes than today, driving upwelling in polar and sub-polar regions of the northern and southern hemisphere. As northern hemisphere glaciation intensified approaching 2.7 Ma, the position of the westerlies shifted towards the equator, leading to a progressive crashing of productivity, first in polar regions at 3.3 Ma to sub-polar regions at 2.7 Ma. As the westerlies moved equatorward, polar and sub-polar waters became more stratified which prevent the mixing of nutrients to the surface. Increased stratification means that less dense water “floats” above denser water, and it therefore is increasingly difficult to move deeper, denser water towards the surface. Although this progressive crash in productivity is best observed in the northern hemisphere, the southern hemisphere is believed to have responded similarly due to the equatorward migration of the westerlies in both the northern and southern hemispheres. As the westerlies moved over the mid latitudes, stronger upwelling occurred over the mid and low-latitude regions, causing an increase in productivity.
The timing of the productivity crashes observed at polar and sub-polar sites are in excellent agreement with additional proxies reported in this study. Increased magnetic susceptibility at 2.5 Ma (Figure 2: a) indicates an increased concentration of magnetic minerals in a sediment core. An increase in more magnetic minerals is ice rafted debris (IRD), which is the process by which sediment scoured by glacial ice is carried offshore by icebergs. As the icebergs melt, the sediment is dropped to the seafloor. For this reason, magnetic susceptibility can be used as a proxy for IRD. Alkenone proxies can be used to reconstruct sea surface temperature (SST) (Figure 2: b). The productivity crash occurring at 2.5 Ma at ODP site 982 (Figure 2) happens concurrently to an increase in IRD and a decrease in SST.
As the westerlies shifted towards the modern day position over the mid-latitudes, increased productivity would be expected at the mid-latitudes, but not necessarily at low latitudes, as the upwelling would be generally confined to the mid-latitudes. However, it is apparent in the proxy records that low latitude sites also experienced increased productivity. The authors of this study produced a conceptual model which helps in understanding increased productivity at low latitudes (Figure 3). The equatorward movement of the westerlies in both hemispheres caused increased meridional (following lines of longitude) SST gradients from the Pliocene to the Pleistocene, caused by cooling of surface waters in polar and sub-polar regions. This caused a contraction of the warm pool which effectively pushed nutrients to the surface at low latitudes (Figure 3). Increased nutrients to low latitude surface waters leads to increased productivity.
The authors concluded this study with implications for deepwater formation, as deepwater formation and global atmospheric CO2 is tightly related. Previous studies conclude that deepwater formation was inhibited by the shift of the westerlies equatorward in the southern hemisphere, since increased stratification at the southern hemisphere polar region inhibits the formation of deepwater. Available data for the northern hemisphere shows continuous deepwater formation in the northern hemisphere polar region. This is largely attributed to the narrow shape of the northern polar basin and the North Atlantic current, which transports warm salty water to the poles. At the poles, this salty water cools, increasing the density and allowing it to sink and form deepwater. The authors emphasize that the reduced ventilation (CO2 rich deepwater upwelled to the surface) of deepwater at the southern polar region and continuous deepwater formation in the northern hemisphere may have contributed to the drawdown of atmospheric CO2 during the Pleistocene.
This study contributes to our understanding of the interactions between intensification of glaciation and productivity. Although there is substantial evidence pointing to a decrease in glacial in our future, it is of utmost importance, especially with the spotlight on human induced climate change to have a robust understanding of the workings of the Earth’s climate system. Understanding how the Earth’s climate system has responded to natural variations betters our ability to model, predict and respond to future changes brought on by increasing atmospheric greenhouse gases.
- Polar and sub-polar productivity crash with onset of northern hemisphere glaciation.
- Mid and low latitude productivity increases concurrent with high latitude crashes.
- Productivity crashes progressive through time, with polar regions crashing first.
- Equatorward migration of the westerlies shifts the position of upwelling, leading to increased polar stratification and increased mid and low latitude productivity.
- Low latitude productivity increases tied to the contraction of the warm pool and shoaling of nutrient rich water to the surface waters.
- Deepwater formation during the Pleistocene in the northern hemisphere did not decrease like the southern hemisphere due to the unique basin and current configurations.
- Reduced southern hemisphere deepwater ventilation and steady northern hemisphere deepwater formation may have contributed to reduced global atmospheric CO2.