Ocean Sediment Core Delineates 100,000 Years of Monsoonal Variation

In 2004, an oceanographic cruise along the Nile delta off of the Mediterranean Sea extracted a 7 m long ocean core from a site 1389 m below the surface. Six years later, Revel et al. (2010) published the data from this core, which provide a record of the past 100,000 years of climate variability and Nile river outflow. Through identifying major elements, calibrating isotopic ratios, measuring particulate grain-size, and collating the data with previously collected paleo-climatic records, the ocean core, known as MS27PT, creates an uninterrupted database of Nile river runoff and the corresponding precipitation rates of the region. The results agree with previously observed records of climatic shifts, and provide higher resolutions than previous studies, typically from about 2–10 years. They also confirm monsoon fluctuation as the primary determinant of river flow, and show that the end of the Nabtian period—a pluvial (rainfall-constituting) era—ended 3 ka earlier along the Nile region than in equatorial Africa. .— Elise Wanger
Revel, M., Ducassou, E., Grousset, F., Bernasconi, S., Migeon, S., Revillon, S., Mascle, J., Murat, A., Zaragosi, S., Bosch, D., 2010. 100,000 Years of African monsoon variability recorded in sediments of the Nile Margin. Quaternary Science Reviews 29, 1342–1362.

The location from which scientists extracted the core—about 100 km from the Nile River mouth—makes the MS27PT core ideal for monitoring the history of the Nile discharge while avoiding the erosion and down-sloping turbidity currents of the Rosetta channel. Thus the core provides an uninterrupted record with no evidence of erosion or displacement from visual examination, X-ray radiography or thin section slices. Three sources of sediment in the core were identified: the Sahara, the White Nile, and the Blue Nile. The Blue Nile sources derive from metamorphic basalt rock younger than 30 Ma, while the White Nile is a granite. The Saharan dust derives from Precambrian granite significantly older than any other source, full of large-grained, quartz particles. Because the parent rocks of all three locations formed at different times and different rates, they each have a distinct strontium (Sr) isotope ratio that makes them easy to distinguish.
Both strontium (Sr) and neodymium (Nd) exist in all crust or mantle earth materials, and come in a variety of stable isotopes, making them effective geological fingerprints from which to determine sediment sources. From these isotopes, scientists have been able to determine that the overflow of Lakes Albert and Victoria that drain into the White Nile began only 11.5 ka BP, and have an almost negligible contribution to the sedimentary discharge during peak flow, due to both the White Nile’s weaker fluvial contribution, and the limited erosion or weathering of the rock material. Lastly, while the Blue Nile receives intense precipitation at higher elevations from the mountains, creating a speedy, downward flow, the White Nile travels along a fairly flat, slow-moving plane, where most sediment would end up sinking in the Sudd swamps of Sudan. These factors make the White Nile’s contribution to the Nile margin typically negligible, and therefore the core can be analyzed in terms of only two sources: Saharan dust and Blue Nile sediments.
The core length (in millimeters), was calibrated into age (in years before present) using a radiocarbon dating of planktonic foraminifera and sapropel events. Sapropels are darkly-colored layers laden with organic carbon that correspond to interglacial periods when rainwater and river runoff are highest. The influx of freshwater into the Mediterranean Sea has a lower salinity which means a lower density, making the incoming water unable to sink and mix with the deep-sea water. Since deep waters can only receive oxygen through circulation and ventilation, this breakdown of the cycle creates anoxic conditions on the sea floor, killing organisms in deep organic matter while simultaneously delaying decomposition. Thus sapropels are preserved organic matter. Sapropel events are well studied and already labeled and dated from previous research. The planktonic foraminifera allowed Revel et al. to confirm these studies as analogous to their core as well as gain a higher temporal resolution. The foraminifera were crushed and their constituents ionized with phosphoric acid, which gives the carbon a charge. Then the ions were separated by accelerator mass spectrometry, which runs them through an electrically charged field at high kinetic energies so that the lighter carbon-12 carrying molecules will move further than the carbon-14. All living material has the same 14C:12C ratio as the atmosphere while alive, but upon dying the organism no longer exchanges carbon with the outside world, and therefore cannot take in more carbon-14. The carbon-14, being an unstable isotope, will decay to the stable carbon-12 at a predictable rate.
Grain-size analysis was calibrated using a laser microgranulometer, which sends out an x-ray and measures the diffraction patterns that deviate from the calibrated expectation of sea water. Element concentrations were similarly determined with an XRF Core Scanner, which recorded images with visible and ultraviolet light waves at three different kilovolt levels. Major elements were also detected and measured through X-ray fluorescence. All these techniques allowed Revel et al. to take pictures of the core without disrupting the integrity of its form, and showed high precision and accuracy to other analyses. The elements evaluated were iron (Fe), sulfur (S), barium (Ba), calcium oxide (CaO), and manganese oxide (MnO).
Oxygen isotope ratios (δ18O) were measured to determine the past climate conditions within the sample. Oxygen comes in three naturally occurring isotopes, with oxygen-16 being the lightest and most prevalent, and oxygen-18 the heaviest and second-most prevalent. Colder or wetter climates (which are not necessarily inclusive) tend to have more oxygen-18 in the water, since the heavier oxygen isotope falls more readily as precipitation and takes more energy to evaporate, while the oxygen-16 will be more likely to freeze in glacial ice. Since calcite, the primary constituent of marine shells, requires oxygen from the environment to form, its oxygen isotope ratio is reflective of the environment in which it was formed, and therefore the calcite of planktonic foraminifera shells in ocean cores can be used to create a δ18O record. The oxygen isotope record can also help determine the contribution of river water to a sample. In being entirely composed of rainwater, river runoff has a lower δ18O than the sea. Yet given the multifarious influences on the δ18O, this record cannot give a complete narrative of the paleoclimate without the auxiliary records of sediments and other elements to complete the picture. A lower ratio tends to be indicative of wetter conditions, but other variables, indicative of sea-surface interactions, need to be considered, such as the productivity of surface biota that may use more oxygen during certain eras, constraining the oxygen that reaches the sea floor, or the if reduced circulation also led to less oxygen access. Revel et al. calibrated the δ18O appropriately for these non-climatic influences, and did not have to worry about the incongruous dichotomy between temperature and humidity, since the region of Africa throughout the last 100,000 years has been much more significantly influenced by climate change concerning moisture, with temperature fluctuations less pertinent to consideration.
Barium and sulfur proved to be a reliable proxy of the organic carbon content, and Revel et al. used the peak of these elements to designate the median of the sapropel thickness. Iron-rich sediments tend to be from the Blue Nile, while carbonate-rich sections—which also have high CaO and a high Si:Al ratio—act as proxies for Saharan aeolian dust. The Fe content strongly correlates to the Sr and Nd isotopes of the Blue Nile basalt as well. Larger grain size, depending on the material, could be correlated to either higher winds if from the Sahara, or higher river current if from the Blue Nile (usually from a mass flooding period).
Most data can be explained with a simple mixing model between the two sources, but the core also reveals distinct periods in which one mode dominates. Saharan dust contributes anywhere from 30–85% as the conditions shift from interglacial monsoon to arid glacial. The rates of sedimentation also shift dramatically from as little as 3 cm/ka to 108 cm/ka during high flood.
These eras of aridity and rainfall, that the Saharan dust and river deposits each respectively indicate, congruously follow monsoonal patterns. As the summer equinox approaches, the Inter Tropical Convergence Zone (ITCZ)—a latitudinal range of wind convergence that forms mass condensation and precipitation—migrates northward to about 20ºN, following the sun’s zenith point and causing a “wet season” (and a dry season at the equator). Given the absorptive and insulating capacity of land, the direct impact of the sun heats the land surface faster and more intensely than the ocean, which is fairly reflective and less susceptible to temperature change. This generates a pressure gradient between the ocean and the land surface (land having the lower pressure due to the warmer air rising), which draws moist, maritime air to the region which quickly rises over the land surface and condenses. Mountains will enhance this cycle by forcing the maritime air upwards and more readily inducing condensation, such as happens in the Ethiopian Highlands, of which Lake Tana and Blue Nile make up the drainage basin.
This monsoonal cycle is indicative of our current climate, but on a millennial timescale this patterns oscillates with the axial precession and orbital eccentricity of the Earth. The Earth rotates along the pole axes in a precessional cycle that takes about 26,000 years to complete, or about 1º per 72 years. This rotation follows a cone-shape with an angle of 23.5º, meaning that the summer and winter equinoxes shift in relation to the changing distance from the sun, and therefore the regions with the most intense sunlight and seasonal contrast move northwards in latitude, and regress back to the equator, every cycle. The orbital eccentricity of the Earth—the shape of it’s orbit—fluctuates from more circular to more oblong in an arguably 100,000 year average cycle, although this varies. When the orbital eccentricity and precession are matched up so that the sun is closest the Northern Hemisphere during its summer season and farthest during its winter—correlating to the precessional minimum and eccentricity maximum—the ITCZ extends the most northward, and the monsoon is the most intense. Conversely, a precession maxima and eccentric minima will create less seasonal variation and less monsoon, which creates an arid and dusty regional climate. The ocean core data accords with these shifts, with a weak monsoon season (which means a precessional minima and minimal ITCZ migration) reflected in a higher oxygen-16 to oxgyen-18 (δ18O), high levels of Saharan dust; and an intense monsoon reflected in a lower δ18O and more Blue Nile sedimentary deposits.
Saharan dust contributions tend to escalate in a snowball effect as more factors enhance the positive feedback of dust flux. The reduced migration of the ITCZ, due to the minimization of the precessional cycle, stimulates a greater thermal gradient between latitudes, causing air to travel as faster speeds as the difference between equatorial climate and the cooler North African region induces a pressure gradient. The dust plumes in the air then keep the North African land surface cool by insulating the area from the sun’s rays and stopping air from rising, through the creation of an inversion—when the air above is warmer than that below—which reduces the monsoonal effect even more, since the monsoon depends on the difference between heated land and the cooler ocean surfaces. The aridity therefore continues and begins to attenuate the soil moisture, making the land less hospitable to plant life which reduces the “savannah-like vegetation” (Revel et al., 2010; 1355) that holds the sediment in place. This cycle is reflected in the record, as Saharan dust gradually builds in concentration during low-monsoon periods. Likewise, precessional maximas correlating with the northwards expansion of the ITCZ reverse this model in the opposite direction, with wet seasons provoking wetter seasons.
Two periods of peak humidity conditions at 34–30 ka BP and 63–50 ka BP are evidenced in the core record. A 6% increased Fe content peaks from 34–30 ka BP, and the period from 63–50 ka BP shows higher organic carbon, as well as higher Fe and Ba, concentrations. Greenland ice cores have found increased methane-levels (CH4) during these same periods, which are usually caused by the circulation and melting of wetlands that have reservoirs of CH4 beneath the surface soil. Revel et al. propose that tropical wetlands during the African monsoonal peak could be the major source pertaining to the ice core record.
Another notable climate event recorded in the core is the end of the Nabtian pluvial period at about 8 ka, dated much earlier than indicated in lake cores from the East Equatorial African region, where rains begin to desist around 5.5 ka. The core shows a shift in the Sr ratio around 8 ka that corresponds to the basaltic parent rock of the Ethiopian highlands, indicating the intensification of an appreciable Blue Nile flow before declining into an arid stage. This transitional period from pluvial to arid from about 14–8 ka caused abnormal weather fluctuations of intense and irregular rains. Revel et al. track this period of “highly variable precipitation intensity” (Revel et al., 2010; 1360) closely along the core at a 2 year resolution. The discrepancy in dating between the MS27PT ocean core and equatorial lake cores most likely denotes the time in which the ITCZ began its gradual reversal southward, instigating increased rains to the Nile region as it passed, but not influencing the lower latitudes of the equatorial climate.
The MS27PT doesn’t only accord with previous paleoclimatic records. It provides a new degree of chronological resolution and reveals the local nuances that solely pertain to the unique trade winds of the North African region, providing a history specific to the Nile River. It also highlights the intricate relationships between systems from macrocosmic astronomical inputs to chemical reactions of benthic microorganisms. As the minima of the precessional cycles intensifies summer monsoon, rainfall increases and with it the freshwater input into the Nile Margin, creating anoxic deep-sea conditions which instigates the formation of sapropel layers that coincide with low δ18O. These interwoven phenomena reinforce each other’s validity by producing a precise and consistent set of variables that all contribute to a coherent story.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s