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CHAPTER 8 • Insolation Control of Monsoons 143

of salty water along the northern margins of the hypothesis (see Figure 8-5). The close match indicates
Mediterranean during incursions of cold air from the some kind of connection to the low-latitude monsoon
north. These two factors make surface waters dense over North Africa.
enough to sink to great depths. The dense waters that Initially some climate scientists questioned this
sink deep into the Mediterranean Sea eventually exit explanation. The Mediterranean Sea lies at high sub-
westward into the Atlantic Ocean. As a result of this tropical latitudes (30°–40°N), beyond even the greatest
flow, the floor of today’s Mediterranean Sea is covered northward expansions of past summer monsoons indi-
by sediments typical of well-oxygenated ocean basins: cated by lake-level evidence across North Africa. If cli-
light tan silty mud containing shells of plankton that mate within the confines of the Mediterranean region
once lived at the sea surface and benthic foraminifera never became truly monsoonal, how could the stinky
that once lived on the seafloor. muds deposited in that basin be a response to the North
Mediterranean sediments also contain occasional African monsoon?
distinct layers of black organic-rich muds, called sapro- The critical link turned out to be the Nile River
pels. Their high organic carbon content indicates that (Figure 8-7), which gathers most of its water from the
they formed at times when the waters at the seafloor highlands of eastern North Africa at tropical latitudes.
were anoxic: they lacked the oxygen needed to convert Even today these highlands receive summer rains dur-
(oxidize) organic carbon to inorganic form. The lack of ing the relatively weak tropical monsoon, and the Nile
oxygen led to stagnation of the deep waters and deposi- delivers the water to the Mediterranean Sea far to the
tion of iron sulfides, giving the sediments a “stinky”
(rotten-egg) odor. The lack of oxygen also kept benthic
foraminifera and other creatures from living on the Deposition
seafloor. 40˚N of sapropels
Paleoecologist Martine Rossignol-Strick proposed
that the deep Mediterranean basin was deprived of
oxygen during intervals when sinking of oxygen-rich 30˚
surface waters was cut off by a cap of low-density fresh- Nile
water brought in by rivers (Figure 8-6B). Even though 20˚ Stronger
the surface waters were still chilled by cold air masses at monsoon
these times, the low-salinity lid kept them from becom- 10˚
ing dense enough to sink deep into the basin. As a
result, the deep Mediterranean basin lost its supply of
oxygen. 0˚
At the same time, production of planktic organisms
continued at the surface and probably even increased as 10˚S
the stronger river inflow delivered extra nutrients 0˚ 30˚E
(food) to the Mediterranean. The high productivity at
the surface continually sent organic-rich remains of Nile
dead plankton toward the seafloor. Sinking and oxida-
tion of this organic carbon continually depleted the
oxygen levels in the deep Mediterranean and produced
the stinky muds on the seafloor.
The most recent sapropel in the eastern Mediter- Libya
ranean dates to 10,000 to 8000 years ago, an interval Egypt
when summer insolation levels were higher than today, Chad
the African summer monsoon was stronger, and African 0 km 50
lakes were at higher levels. Earlier layers of organic-rich Sudan
mud deeper in Mediterranean sediment cores occur FIGURE 8-7 Monsoons and the Nile River Strong summer
at regular 23,000-year intervals during times when monsoons in tropical North Africa periodically produced large
summer insolation was higher than it is today. The discharges of Nile freshwater into the Mediterranean Sea.
sapropels were best developed (thickest and most Satellite sensors have detected riverbed sediments deposited
carbon-rich) near the time of the strongest summer during strong monsoons but now buried beneath sheets of
insolation maxima, but they were poorly developed dur- sand in the hyperarid eastern Sahara Desert (inset). (Inset
ing weaker insolation maxima and were absent the rest adapted from H.-J. Pachur and S. Kroplein, “Wadi Howar:
of the time. This history of sapropel deposition matches Paleoclimatic Evidence from an Extinct River System in the
very well the pattern predicted by the orbital monsoon Southeastern Sahara,” Science 237 [1987]: 298–300.