Environmental Engineering Reference
In-Depth Information
13
C values (e.g. [4, 34, 74]). Well preserved
organic matter allows the detailed reconstruction of primary productivity and
secondary mineralization of sedimentary organic matter. For the lower Jurassic
Toarcian black shales of Europe for example, organic geochemical data (bio-
marker, compound-specific organic carbon isotope data) indicate oscillations
in the depth of the chemocline suggesting recurring periods of anoxygenic
photosynthesis (e.g. [75, 77]).
by shifts to positive carbonate δ
4.5 Neogene Mediterranean Sapropels
The Mediterranean Neogene sedimentary record contains abundant exam-
ples of centimeter to decimeter thick dark laminated layers containing up to
30% organic carbon (for review see e.g. [68]). These so called sapropels were
deposited most prominently during Pliocene and Pleistocene time, predom-
inantly but not exclusively in the eastern Mediterranean (Fig. 1). Sapropel
formation is thought to be a consequence of sluggish deep water ventilation
as a result of a stable pycnocline which affected much of the late Miocene to
early Holocene Mediterranean Sea. The latter was induced by climate-driven
enhanced river flow from Eurasia and Africa into the Mediterranean causing
a strong salinity difference. Strongly contrasting views exist with respect to
surface water nutrient availability and resulting levels of primary productivity
(e.g. [16, 72]). Subsequent degradation of sinking organic matter, in addition to
the salinity gradient, forced the establishment of a stagnant deep ocean. It has
been proposed that climate-driven sapropel formation is ultimately linked to
variations in the precession, which occur about every 21000 years (e.g. [69]).
Apart from the distinct lithology, ample biogeochemical and isotopic evi-
dence exists for the sapropel formation (e.g. [15, 54-56]), but also extending
into the photic zone [53].
5. SUMMARY
Multiple times in Earth history, anoxic conditions were prevailing in large
parts of the ocean. The occurrence of widespread, possibly global oceanic
anoxic conditions is a consequence of different factors interrelated with each
other including the evolving oxygenation of Earth's surface environments, cli-
mate-induced differences in oceanic circulation and increased primary produc-
tivity causing an enhanced oxygen demand in the water column. The temporal
record of water column anoxia is resolvable to different levels of detail, rang-
ing from globally representative long-term seawater isotope records of carbon,
sulphur, and to a lesser degree of nitrogen, to organism-specific molecular
information and compound-specific organic carbon isotope work.
The temporal record of anoxia suggests a largely anoxic Precambrian world
in which the oxygenation of the atmosphere and the surface ocean occurred
