5 Discussion
5.3 The size of the drainage area as an indicator for the terrestrial sediment supply
The drainage area of the Vestfjord, Ofotfjord, Tysfjord, and tributary fjords has a total extent of 7 118 km2 (Fig. 8). The drainage area/fjord surface area ratio is calculated as 47.23% for the fjord surface and 52.77% for the land. The catchment area and thus the contribution of terrigenous derived material into the fjords are strongly influenced by the steep mountain ranges of the Lofoten archipelago and by the mountains in the southern parts of the area as size and topography of a drainage area primarily control the sediment discharges of most rivers (Milliman and Syvitski, 1992). The high topographic, mountainous character of the area leads to the inhomogenous distribution of the sediments in the entire area (see chapter 5.1) as effective mixing of source rocks and soils is strongly limited by the short transport distances (Gaillardet et al., 1999). In addition, the catchment area is characterized by several small rivers entering the fjords. In contrast, the Trondheimsfjord drainage area (Fig. 49), for example, is defined by six large rivers entering the fjord (Sakshaug and Sneli, 2000; Faust et al., 2014b). Those rivers are not restricted by such high mountains and hence have relatively long transport distances following in a more distinct distribution pattern with increasing grain sizes from the inner part of the fjord towards the outer part (Faust et al., 2014a).
74 Figure 49: Location and drainage area of the Trondheimsfjord. Modified after Rise et al., 2006.
The drainage area for the Trondheimsfjord has a total extent of 20 000 km2 (Rise et al., 2006).
The size correlates with a drainage area/fjord surface area ratio of 92.9% for the land and 7.1%
for the fjord surface. By comparing the Trondheimsfjord and the fjords of the study area with each other, it is evident that the size of the fjord surface area in the study area is relative larger in contrast to its drainage area than the fjord surface area of the Trondheimsfjord compared to its catchment area. The relation between fjord and land area is almost 1:1 in the study area illustrating that the drainage area has nearly the same size as the fjords (Fig. 8). This leads to a relative low contribution of terrigenous material as the high amounts of marine material produced by marine organisms decrease the effect of the terrestrial matter. Thus, terrigenous material has an extraordinary small impact on the general sedimentary content of the surface samples. In contrast, the Trondheimsfjord drainage area/fjord surface area ratio indicates relative higher supply of terrigenous material due to the small proportion of the fjord surface area compared to the catchment area, and hence the relatively small contribution of MOM.
75 Therefore, terrestrial derived sediment input into the fjords of the study area is comparatively low compared to both - the supply of terrestrial matter into the Trondheimsfjord and the amount of marine derived material that is produced in the fjords of the study area. The observations are consistent with the general relatively small contribution of terrigenous material illustrated by Ninorg and the bulk inorganic material as well as by the high amount of MOM shown by Corg/Norg, δ13Corg and δ15Norg (see chapter 5.2).
The North Atlantic Oscillation (NAO) which describes the most robust periodical mode of atmospheric circulation variability in the North Atlantic region and the Norwegian Atlantic Current (NAC) both have strong influences on the Norwegian coastal climate (e.g. Hurrell, 1995; Dickson et al., 2000). Hurrell (1995) reported an effect of the NAO on the transport and convergence of atmospheric moisture which further can be related to changes in precipitation on land. As the NAO and the inflowing NAC are present at the Norwegian coast and thus also in the fjords of the study area, precipitation, temperatures and wind intensities most likely are effected by variabilities in atmospheric pressure and by the inflowing Atlantic water masses of the NAC and Norwegian Coastal Current (e.g. Hurrell, 1995).
Precipitation in the study area ranges between 855 mm/year for Narvik and 1 500 mm/year for Svolvær (Norwegian Meteorological Institute, 2015) and is responsible for river runoff and hence supply of terrigenous material to the fjords. In addition, annual mean temperatures vary from 3.6°C for Narvik to 4.6°C for Svolvær (Norwegian Meteorological Institute, 2015). Both precipitation and temperatures are in the range of those for the Trondheimsfjord region with annual mean precipitation of 1 100 mm/year and average winter temperatures of -4.6°C (Faust et al., 2014b). Faust et al. (2014b) reported that changes in the regional temperature, precipitation and river runoff are strongly related to variabilities of the NAO. As temperature and precipitation strongly control the intensity of terrigenous weathering and erosion (e.g.
Syvitski, 2002), the contribution of terrestrial derived material has been reported to be able to record past NAO changes in the Trondheimsfjord (Faust et al., 2014b). Given the similar characteristics of both areas, the reconstruction would also be possible in the study area by using longer core samples.
In addition, temperature and precipitation of the study area have relatively small values compared to Patagonian fjord systems (Silva et al., 2011; Sepúlveda et al., 2011; Bertrand et al., 2012) where the mean annual precipitation is 3 000 mm/year with a mean annual temperature of 10°C (Sepúlveda et al., 2011). Inner parts of Patagonian fjords are surrounded
76 by dense vegetation characteristic of cold, wet climate regimes in the form of temperate evergreen forests (e.g. Villagrán, 1988). This also is in contrast to the fjords of the study area, where the vegetation cover is relatively sparse and dominated by periglacial mountain plants (e.g. Vorren and Moe, 1986). Generally, high precipitation and warm temperatures are strongly connected to relative high rates of chemical weathering (Nesbitt et al., 1996). Thus high erosion rates and meltwater discharges in spring and summer lead to the increase of terrigenous sediment supply to river systems and marine environments (e.g. Pollack, 1999).
Therefore, precipitation, temperature and short transport distances in the study area are parameters most likely influencing the terrestrial sediment supply into the fjords (e.g. Hurrell 1995; Syvitski, 2002). This leads to the comparatively weak contribution of the terrigenous fraction as precipitation and temperatures are relative low (Norwegian Meteorological Institute, 2015) and the vegetation cover is sparse compared to other fjord systems (e.g. Sepúlveda et al., 2011). Investigations concerning the precipitation and annual mean temperatures are consistent with the overall low terrestrial sediment supply observed by the contribution of the bulk elemental composition which is extremely low (see chapter 5.1) and by δ13Corg, δ15Norg, and Corg/Norg which present an overall high supply of MOM compared to the terrestrial matter.
However, size and topography of the drainage area are of more importance controlling sediment discharges than net precipitation and river runoff that play a minor role (Milliman and Syvitski, 1992).
Moreover, the presence of the three main glaciers (Kitjejekna, Storsteinsfjellbre, Frostisen) in the area (Fig. 10) probably could contribute to increased terrestrial sediment supply (e.g. Greve and Blatter, 2009 and references therein). This likely can be illustrated by comparing sample stations from the inner fjord part close to the glaciers (e.g. stations 4, 23, 24, 25) with other inner fjord stations that are located further away from the glaciers. As there is no significant difference in concentrations of the bulk inorganic material, it is assumable that the influence of the glaciers is not significant due to short transport distances and the restricted area of the glaciers.
In summary, the contribution of terrestrial material into the fjords of the study area is relatively small compared to the supply of marine derived material. This may be due to the size and the topography of the drainage area which are most influencing on sediment discharges (Milliman and Syvitski, 1992). Thus transport distances are strongly limited by the mountainous character.
Precipitation and temperature are also comparatively low compared to other fjord systems (e.g.
77 Sepúlveda et al., 2011) leading to decreasing rates of weathering and erosion. However, precipitation, temperatures and river runoff are of minor influence for the sediment supply (Milliman and Syvitski, 1992) as only 52.77% of the area are defined by land compared to 47.23% for the fjord surface area. Hence, the percentage of terrestrial material entering the fjords is decreased compared to the contribution of marine material.
The observations are in contrast to the overall strong mixing between terrestrial and marine sources in coastal environments which has been observed by Peters et al. (1978), Knies (2005), Wada et al. (1987). Moreover, Schubert and Calvert (2001), Winkelmann and Knies (2005), Sepúlveda et al. (2011), Silva et al. (2011), and Faust et al. (2014a/b) reported a relatively balanced contribution of both terrestrial and marine material into sediment samples. This is contrary to the sediment samples of the study area since those are highly dominated by MOM compared to the low contribution of terrigenous material resulting in an imbalanced contribution between marine and terrestrial material.