188 Imprint of Climatic Zonation on Marine Sediments
tion rate of calcareous, siliceous, and phosphatic materials. Also, certain metals are
concentrated below the equator, by the upwelling mechanism. We shall return to
these subjects when discussing deep-sea sedimentation (Chap. 8) and ferromanganese
nodules (Sect. lOA).
Nutrient supply, as already mentioned, is increased through vertical mixing and
upwelling. Upwelling is highly seasonal, being induced by the wind field (Fig. 4.19),
which is quite variable, of course. Thus, the major upwelling regions - off Peru,
California, northwestern Africa, Namibia, and also in the Arabian Sea - show high
but greatly varying productivity.
Seasonal upwelling is largely tied to monsoonal activity. On the continents, seasonal contrasts are exaggerated, compared with the oceans (continental climate).
Around the large and high land masses of East Asia, this seasonal contrast affects the
entire northern Indian Ocean (monsoonal climate, see Fig. 7.2, hachured area). During winter, cold air masses flow off the continent (northeast monsoon in India)
bringing drought and upwelling. During summer, the circulation is reversed: the
southeast monsoon brings rain from the ocean. In the eastern Pacific, upwelling
virtually ceases every 3 or 4 years for a year or so. El Nino conditions are said to
prevail during such events. "EI Nino" means "the child" and refers to the Christ child,
in reference to the observations, by Peruvian fishermen , that conditions of poor
fishing start around Christmas. The El Nino phenomenon is not well understood,
although it is of great importance even on a global scale. The history of such oscillations can be studied in sediments with annual layers (varves) off California (Fig.
7.17), off Peru, and in the Gulf of California. In the Santa Barbara Basin off Southern
California, El Nino layers are characterized by unusually large proportions of warmwater diatoms, and reduced supply of organic matter. Because of this reduced supply,
oxygenation increases, and in some cases bioturbation sets in, destroying a number of
varves. Thus, the stratigraphy of bioturbation in the Santa Barbara Basin contains
valuable clues to the long-term workings of the Pacific wind field, and indeed the
global heat budget.
7.2 Biogeographic Climate Indicators
7.2.1 Paleotemperature from Transfer Methods. Biogenous sediments are excellent indicators of climate: many organisms have narrow ranges of temperature and
fertility and the presence of their remains immediately yields excellent clues for
climate reconstruction. Also, the chemistry of the shells can give indications of temperature, salinity, and growth rate. The most widespread biogenous deposit on the
present sea floor is calcareous ooze, which consists of plankton shells. Of the shellmaking plankton, the foraminifera were found to be the climate indicators par excellence. They are reasonably diverse, widespread, and easily identified under a lowpower microscope (that is, after some practice). Planktonic foraminifera live mainly
in the uppermost 200 m of the water column. There are about 20 abundant species,
and as many rare ones. Each climatic zone has one or more characteristic species and
a typical abundance pattern of species within it. Some temperature-sensitive species
tion rate of calcareous, siliceous, and phosphatic materials. Also, certain metals are
concentrated below the equator, by the upwelling mechanism. We shall return to
these subjects when discussing deep-sea sedimentation (Chap. 8) and ferromanganese
nodules (Sect. lOA).
Nutrient supply, as already mentioned, is increased through vertical mixing and
upwelling. Upwelling is highly seasonal, being induced by the wind field (Fig. 4.19),
which is quite variable, of course. Thus, the major upwelling regions - off Peru,
California, northwestern Africa, Namibia, and also in the Arabian Sea - show high
but greatly varying productivity.
Seasonal upwelling is largely tied to monsoonal activity. On the continents, seasonal contrasts are exaggerated, compared with the oceans (continental climate).
Around the large and high land masses of East Asia, this seasonal contrast affects the
entire northern Indian Ocean (monsoonal climate, see Fig. 7.2, hachured area). During winter, cold air masses flow off the continent (northeast monsoon in India)
bringing drought and upwelling. During summer, the circulation is reversed: the
southeast monsoon brings rain from the ocean. In the eastern Pacific, upwelling
virtually ceases every 3 or 4 years for a year or so. El Nino conditions are said to
prevail during such events. "EI Nino" means "the child" and refers to the Christ child,
in reference to the observations, by Peruvian fishermen , that conditions of poor
fishing start around Christmas. The El Nino phenomenon is not well understood,
although it is of great importance even on a global scale. The history of such oscillations can be studied in sediments with annual layers (varves) off California (Fig.
7.17), off Peru, and in the Gulf of California. In the Santa Barbara Basin off Southern
California, El Nino layers are characterized by unusually large proportions of warmwater diatoms, and reduced supply of organic matter. Because of this reduced supply,
oxygenation increases, and in some cases bioturbation sets in, destroying a number of
varves. Thus, the stratigraphy of bioturbation in the Santa Barbara Basin contains
valuable clues to the long-term workings of the Pacific wind field, and indeed the
global heat budget.
7.2 Biogeographic Climate Indicators
7.2.1 Paleotemperature from Transfer Methods. Biogenous sediments are excellent indicators of climate: many organisms have narrow ranges of temperature and
fertility and the presence of their remains immediately yields excellent clues for
climate reconstruction. Also, the chemistry of the shells can give indications of temperature, salinity, and growth rate. The most widespread biogenous deposit on the
present sea floor is calcareous ooze, which consists of plankton shells. Of the shellmaking plankton, the foraminifera were found to be the climate indicators par excellence. They are reasonably diverse, widespread, and easily identified under a lowpower microscope (that is, after some practice). Planktonic foraminifera live mainly
in the uppermost 200 m of the water column. There are about 20 abundant species,
and as many rare ones. Each climatic zone has one or more characteristic species and
a typical abundance pattern of species within it. Some temperature-sensitive species