Dust Particle Accumulation Shows Glaciations and Orbital Forcing Theory Disproved

The Northern Hemisphere Glaciation (about 2.7 Ma) is one of the more important changes in Earth’s climate. The characteristics first demonstrated during that time have influenced the future patterns of glaciations. Thus understanding that time period could be pivotal in understanding the climate today. Naafs et al. (2012) attempt to evaluate dust particle levels using sedimentary cores in the northern North Atlantic. Dust may play an important role during glaciations, and has been seen to increase during glacial cycles in other parts of the world. However, no work has been done in the North Atlantic and North American region. The authors also want to evaluate the origin of the dust particles, and do so using long-chained alkanes to ensure it is coming from North America. Long-chained alkanes originate from organic plant material, which gets carried off by wind gusts. The alkanes are predominantly found in C3 plants, which represent most of the foliage in North America. C4plants dominate other potential sources of dust, specifically Africa and Eastern Asia. This allows the authors to confirm that North America is the main source of dust at their drill site in the North Atlantic.
Having determined that dust particle levels greatly increase for the first time during the Northern Hemisphere Glaciation, and that the particles originate from North America, the authors test an orbital forcing hypothesis for the first time. It has been proposed that precession forcings drive glacial cycles on a 23 and 19 thousand year cycle (ky), but the affects are cancelled out because of the effect it has on both hemispheres. Using the data collected at the drilling site (i.e. dust build up and oxygen-18, both proxies for glaciation), Naafs et al. tested to see the effect precession had on glaciation. They discovered that in the North Atlantic the frequency of change was driven mostly by a 41 ky cycle, or the obliquity forcing. Naafs et al. call for a new hypothesis to be proposed to help explain why current theory suggests precession has a larger forcing on glaciations. –Mathew Harreld
Naafs, B.D.A., Stein, R., Hefter, J., Acton, G., Haug, G.H., Martínez-Garcia, A., Pancost, R., Stein, R. 2012. Strengthening of North American dust sources during the late Pliocene (2.7 Ma). Earth and Planetary Science Letters 317-318, 8–19.

            Changes in the earth’s climate toward the Northern Hemisphere Glaciation (NHG) during the Pliocene-Pleistocene period (2–4 Ma) were heavily driven by changes in the North Atlantic Current. It has also been noted that large amounts of dust particles (i.e. organic matter collected by winds) are present in glacial stages than in interglacial stages since the NHG. Work in Antarctica has revealed a trend in large amounts of dust deposits during glacials, but virtually zero work has been done in the northern North Atlantic region to evaluate this possible trend. The presence of dust particles may have a large impact in the understanding of glacial cycles, and help understand the natural cycles that drive such changes. Changes in dust concentrations may have a large effect on the presence of CO2 in water, the formation of clouds, and ice albedo (reflectivity). Naafs et al. evaluate dust levels in the northern North Atlantic in an attempt to better understand the NHG. Improving our understanding of how dust might play a role glaciations will greatly increase our ability to potential predict future changes in our current climate. In previous work, Naafs presented the evidence for the change in the North Atlantic Current during and before the NHG, but now explores why this change may have occurred.
            Naafs et al. used a drilling site in the middle of the Atlantic Ocean for their data, the same drilling data used in previous papers. Using four different drilling holes, the dust data were acquired and evaluated with a variety of methods. Oxygen-18 records compiled from previous work (Naafs et al. 2010, 2011) were used by the current authors to measure dust age and quantity. Naafs et al. also established a method for calculating if dust particles were from two types of plants, C3 plants and C4 plants, using long-chained n-alkanes and n-alkan-1-ols, substances that make of plant tissue material. Due to the different methods of photosynthesizing Naafs et al. were able to determine whether the dust collected originated from C3 plants or C4 plants by the level of n-alkanes and n-alkan-1-ols found in the sediment cores. The importance of this determination is in the geographic location of the two types of plants. C3 plants tend to be in grassier and wetter areas—like temperate North America—whereas C4 plants tend to be in drier areas. The accumulation rates of the n-alkanes and the n-alkan-1-ols were also calculated, allowing the authors to test the variability of dust presence and glaciation cycles.
            The dust collected in the coring samples was almost certainly from the North American Continent. Using Carbon Preference Index (CPI) Naafs et al. were able to determine that the dust contained in the cores was closest matched by dust collected in North America, and the closest match was found in Bermuda. Furthermore, the dust carbon-13 levels suggested C3 plants were most continuous and common source, which rules out dust from Africa or East Asia, since both are dominated by C4 plants.
            The results showed a low accumulation rate of dust during the late Pliocene (i.e. before glaciation). About 2.7 Ma there begins a significantly large increase in dust accumulation, right in accordance with the beginning of major Northern Hemisphere glaciation. However, during the interglacial periods the amount of dust greatly decreased, as compared to glacial periods. This seems to show a great correlation between dust and glaciations. Furthermore, the comparison of the North Atlantic dust record to the Southern Ocean record shows the two are greatly similar in trend, differing only in variation of total amounts. This shows further support for the relationship between glacials and increased dust and, thus, wind levels.
            The suggestion that wind levels must have increased however is misleading. An increased wind force during glacials does not support the necessary amount of dust seen in the data. Instead Naafs et al. suggest that the 30-fold increase of dust during the glacial periods may be explained by the increased glacier margins on continents resulting in more land erosion, meaning more dust is brought out to sea. The larger variation in ice extent on North America during glacial and interglacial events may have driven the larger increase in dust sources during glacial events.
            Having determined that dust sources during glacial periods only began around 2.7 Ma, and that the pattern follows for the rest of the cycles, as well as the cycles observed in the higher southern latitudes, Naafs et al. decided to take the opportunity to test a relatively untested hypothesis. A few scientists decided to try and explain Milankovitch cycles, which stated that climate variation was mainly driven by orbital precession. However, during the Pleistocene, ice volume varied on a cycle closer to the obliquity period. The scientists proposed that precession did in fact drive changes in climate, but that it affected both hemispheres, and thus cancelled each other out. Naafs et al. were able to test this hypothesis for the first time because of their work on creating a long and continuous climate record for North Hemisphere ice-sheet variability, using a cross-spectral analysis of oxygen-18 and dust concentrations. The results of the cross-spectral analysis showed that changes in ice-variability were on the scale of 41,000 years (41 ky), or obliquity cycle, which corresponds with the data they collected at their site. If precession cycles drove ice volume then evolutionary change of the n-alkanes should be similar to the precession driven ice volume, since more n-alkanes were present as dust during glaciations. However, Naafs et al. found that the n-alkanes varied on a 41 ky scale, not the predicted 23 and 19 ky scale of precession. This finding suggests that precession cycles have little to no effect on ice-sheet volume during the Pleistocene. This finding is also supported by their other data, pushing the authors to call for a new hypothesis to be developed. 

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