Implications of the orbital runs

9.7.1. River discharge and its effect on the freshwater balance of the WIS

The even annual distribution and the low sensitivity to changes of the precession of the equinoxes indicates that there is probably no connection between subsurface runoff and the cyclic formation of bedding couplets. However, the effect of subsurface runoff on the fresh water balance of the WIS is very large compared to the effect of surface runoff, because the mean annual subsurface runoff is six to seven times larger than surface runoff. The annual distribution shows that

subsurface runoff in the southern part (~4.4 mm/day) of WNA is about 0.7 mm/day higher than in the northern part (~3.7 mm/day). River discharge (subsurface- plus surface runoff) from WNA is

~3,757 km³/yr. About 1,974 km³/yr come from the northern part and ~1,863 km³/yr from the southern part. This large constant input of freshwater from WNA and the inflow from the Arctic Ocean (Figure 102) was probably adequate to establish a permanent low salinity lid on the WIS, at least over the northern part.

Area of water

surface; [km ]² Area of drainage

basin; [km ]² River discharge

; [km /yr]³ Volume of water

mass; [km ]³ Thickness of freshwater added

WISnorth only 5,459,862 1,291,667 1,974 1,364,966 ~0.36

Arctic Ocean

present day 14,700,000 22,350,000 5,140 16,700,000 ~0.35

WISsouth only 6,092,119 1,008,848 1,863 1,523,030 ~0.31

Table 5: Comparison of the present day Black Sea and the Cretaceous WIS (VLADIMIROV, 1999;

ALYUSHINSKAYA and IVANOV, 1978).

A modern analog to the northern part of the WIS might be the Black Sea. Although it lies in a region where evaporation exceeds precipitation, its fresh water balance is dominated by the large

land area and volume of water which drains into it through a few rivers (50% from the Danube alone). The fresh water balance is responsible for freshening the surface layer to about 17$ whereas the deep waters are about 35$. These salinity differences control the Black Sea marine ecosystem. Changes in the fresh water input have a significant impact on salt and water balances, particularly in the shallow, biologically highly productive north-western region. Low salinity surface outflow from the Black Sea and high salinity bottom inflow of Mediterranean water through the Bosphorus are controlled by the width and depth of the strait, and are critical elements maintaining the unusual hydrography and ecosystems of the Black Sea. The fresh water balance of the Black Sea is usually negative from July to October when river inflow is minimal, but reverses during the rest of the year. In some years the fresh water balance is also negative in January, June,

November, and December. The strongest positive balance of fresh water occurs from February through May. The total annual river discharge into the Black Sea (353 km³/yr) is ~1/10 of that of the WIS, equal to a layer 0.77m thick. The area of the drainage basin (1,874,904 km²) is about 20%

smaller than that of the WIS. The volume of the Black Sea is ~547,000 km³ compared to the

~875,000 km³ of the WIS.

Another modern analogue might be the Arctic Ocean, which is not as restricted from the open ocean as the Black Sea. There are major ocean water fluxes through the Greenland-Spitsbergen gap, the Bering Strait, and the Arctic Archipelago. Salinities of the open Arctic Ocean range

between 28-35$, but are less on the shelves. The inflow of surface- and subsurface runoff add the equivalent of a ~0.35m thick layer of freshwater to the Arctic Ocean each year. The Arctic Ocean is characterized by a large seasonal cycle of fresh water input against a background of climatically significant interannual and decadal timescale variations. Today, the Arctic Ocean has an area of 14,700,000 km² and river discharge of ~5,140 km³/yr. This would a add layer with a thickness of 0.35m each year.

River discharge from WNA into the WIS is simulated to be the equivalent to a layer about 0.32m thick, about half of that of the Black Sea. This large inflow suggests that a reduced salinity layer would have existed in the WIS if the connections to the open ocean were restricted. The northern connection to the open ocean was through the Arctic Ocean which received major fresh water inflow from eastern N-America, Europe and Asia. During Cenomanian/Turonian times, the Arctic Ocean had an area of at most 3 x 10⁶ km². Then, as today, a large part of Asia, Europe,

Greenland, and North America drained into it. Surface runoff into the Arctic Ocean was simulated to be 3,375 km³/yr for orbital case A, 5,054 km³/yr for orbital case B, 4,962 km³/yr for orbital case C, and 4,727 km³/yr for orbital case D. Annually, these volumes of surface runoff should have added thicknesses of 1.10m, 1.70m, 1,65m, and 1,60m of fresh water to the Arctic Ocean. The subsurface runoff is not yet included in those numbers. These constant very large fresh water inflows would easily be able to maintain a salinity below 25$, as suggested by FISHER et al.

(1994). Due to higher river discharge from northern WNA and exchange with the low salinity Arctic

9. GCM-modeling 184

Ocean, the low salinity surface waters were probably thicker in the north. The lack of calcareous benthic foraminifera indicates that the low salinity waters reached the floor of the seaway.

9.7.2. Mechanical erosion and sediment discharge by rivers

Surface runoff from land is responsible for the mechanical erosion of terrigenous detritus. It is the surface runoff which erodes loose material and provides the suspended and fraction load of rivers.

The major factors controlling erosion are: a) area of the drainage basin, b) large-scale relief, c) local relief, d) geologic conditions in the drainage basin, and e) climate (SUMMERFIELD and HULTON, 1994; ALLEN, 1997; HAY, 1998; HOVIUS, 1998; HARRISON, 2000). Vegetation also plays a major role; in a frequently cited paper, LANGBEIN and SCHUMM (1958) noted that the spacing between plants and degree of exposure of the soil to precipitation are important factors in erosion and surface runoff. They indicated that the most rapid erosion occurs where precipitation supports only sparse plant cover, so that bare soil is exposed between the plants and roots do not bind all of the soil. Unfortunately, the model runs described here did not include an interactive vegetation model, so this effect can not be evaluated.

The sedimentary significance of the concentration of precipitation into short periods of time was first recognized by FOURNIER (1960), who proposed that an uneven distribution of precipitation through the year increases erosion. The Oxford Sediment Flux Database (ALLEN, 1997; HOVIUS, 1998) shows that this also true for surface runoff.

Another important parameter in determining the amount of sediment delivered to the sea is the competence of rivers. Competence refers to the largest particle a river can carry in suspension, and is indicative of the total amount of sediment it can carry. Increased flow in a river enhances its ability to carry large amounts of sediment downstream. Competence is high when the river

transports large volumes of water and when its gradient is high.

Cretaceous rivers were very different from those of today. Today, about 2/3 of the amount of water of a river comes from surface runoff and only 1/3 from the groundwater system. The model

simulations indicate that during the Cretaceous the ratio of surface runoff/subsurface runoff varied between 1:6 and 1:14 during the 4 orbital models. Totally different from the 2:1 ratio observed today. Only ~7-17% of the river discharge is contributed by surface runoff, but it was surface runoff that generated the suspended load of the rivers. This study has shown that surface runoff from WNA is strongest in June, July, and August. This concentration of runoff, due to increased occurrence of intense convective storms would have resulted in increased mechanical erosion of detrital material on land. Due to the constant large volume river discharge throughout the year, competence of the Cretaceous rivers must have been high, and they would have been able to transport any detrital material they received into the seaway.

Bedding couplets could only form where variations in the surface runoff regime took place and where the sensitivity to orbital forcing was high.