Analyzing five box cores at the Royal Netherlands Institute for Sea Research
with the use of XRF core-scanner, grain size analysis and
210Pb measurements
Sediment accumulation rates in the
Norfolk canyon, West Atlantic ocean
Fleur van Crimpen 10345442 Future Planet Studies Major Earth sciences University of Amsterdam
Supervisor: Dr. Furu Mienis Royal NIOZ Second reader: Dr. Boris Jansen UvA June 2015
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The research for this Bachelor Thesis was carried out at the Royal Netherlands
Institute for Sea Research (NIOZ), Department of Marine Geology and Chemical
Oceanography, P.O. Box 59, 1790 AB Den Burg, The Netherlands
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Index
1. Abstract
p.6
2. Introduction
p.8
2.1. Submarine canyons
p.8
2.2. The Norfolk canyon
p.9
3. Methods
p.10
3.1. Field sampling
p.10
3.2. Core preparation
p.10
3.3. X-Ray Fluorescence core scanner
p.11
3.4. Grain size analysis
p.11
3.5.
210Pb analysis
p.12
4. Results
p.14
4.1. Sediment texture and composition
p.14
4.2. Porosity
p.16
4.3. Grain size distribution
p.18
4.4. XRF analysis
p.20
4.5.
210Pb analysis
p.21
5. Discussion
p.24
6. Conclusion
p.27
7. Evaluation and acknowledgement
p.28
8. References
p.29
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1. Abstract
Submarine canyons can be found on all continental margins, the Mid Atlantic Bight (MAB) located in the West Atlantic Ocean has thirteen major canyons, one of them is the Norfolk canyon. Due to re-suspension there is a variability in accumulation rates in the Norfolk canyon. For this research five box cores were analyzed which were collected during the research cruise RB13-03-HBH Deepwater Canyons Research Cruise 2013, with the NOAA vessel Ronald H Brown. The sediment samples from the Norfolk canyon were analyzed at the Royal NIOZ with the use of XRF analysis, grain size distribution and 210Pb analysis in order to obtain information about accumulation rates. Results shows significant different accumulation rates in the canyon compared to the shelf. It can be stated that accumulation rates are strongly influenced by the complex interaction between topography of the canyon floor and present currents and internal waves.
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2. Introduction
2.1. Submarine canyons
Canyons differ in shape and size, from gullies to huge v-shaped canyons which can inhabit many different zones according to de Stigter et al (2007). The formation of submarine canyons happens due to erosion processes and sediment slides which can be activated by internal currents (Obelcz et al. 2013) and mass wasting (de Stigter et al. 2007) and are formed during low sea-level stands. This due to direct deposition of fluvial and littoral material from the continental shelf (Obelcz et al. 2013 which causes the continental shelf to erode and form gullies.
In earlier research it was stated that canyons are mostly presented as conduits for unidirectional transport of material from shallow water to the deep sea (Gardner 1988). But according to de Stigter et al. (2007) knowledge of timescales on which sediment is transported through the canyons is not only interesting for geological aspects but it will also provide more information about the element cycling in the ocean. This can be obtained by analyzing the cores for different present elements. According to de Stigter et al (2007) internal tides and sediment gravity flows which occurs in episodes are main processes in transporting sediments through the canyon. Internal waves and a complex system of tidal forcing, wind events and differences in density causes re-suspension and causes a flux in sediment transportation within the canyon (CSA International, Inc. 2010). Internal waves are often generated along the edges of the canyon and are the main driver of re-suspension of shelf sediments (de Stigter et al. 2007 & Quaresma et al. 2007). Topography of the ocean floor contributes to the biodiversity, this is probably due to the sediment flow which is enriched in nutrients and suspended organic matter (Gartner et al. 2008). Most of these biodiverse aggregations are visible along steep (>60%) ridges or canyons, currents flow over them and provide needed nutrients for zooplankton which increase significant in this area’s (Vetter et al. 2010). When more is known about the complex interaction between topography of the canyon floor and internal sediment flows, understanding of aggregation patterns and element cycling in the ocean can be improved.
Submarine canyons can be found on all continental margins (de Stigter et al. 2011), the Mid Atlantic Bight (MAB) region, located in the West Atlantic Ocean has thirteen major canyons. One of them is the Norfolk canyon (CSA International, Inc. 2010), located at the most southern point of the MAB and with the canyon head located approximately 100 km from the coast. In order to obtain more information about the sediment accumulation processes in the Norfolk canyon three questions will be answered in this thesis.
What are the sediment accumulation rates in the five different cores taken in the Norfolk canyon.
What are the particle transport processes in the Norfolk canyon which causes the difference in accumulation rates.
How can distinctive zones within the Norfolk canyon be described and what are the main processes which causes this difference .First an introduction will be given in the main features of the Norfolk canyon in order to have a better understanding of the main processes and external influences. After that the methods and materials which are used for this research will be explained so the procedures can be validated and repeated if necessary. The obtained data will be presented in the result section and discussed in the context and the relation to the Norfolk canyon in the discussion. In the conclusion the main findings will be presented and the answers of the three main questions will be summarized. As last a short and personal evaluation of the whole process is given.
9 2.2. The Norfolk canyon
The Norfolk canyon is the second best studied canyon of the MAB, the Norfolk canyon cuts 16 km into the continental shelf and slope (Forde et al. 1981). The canyon is oriented East-West, the north-east wall exists out of massive sandstone cliffs, in the whole canyon the northern wall is steeper and more rough shaped compared to the southern wall (Obelcz et al. 2013). The Norfolk canyon has a sigmoidal shape and has a broad axial bend 9-10 km seaward of its head, the width of the canyon floor varies from 250 m at the head to 1000 m (Table 1).
Table 1. Main characteristics Norfolk canyon (Obelcz et al. 2013).
Figure 1. (A) Norfolk canyon indicated with the yellow pin (google earth). (B) Cross-section bathymetric profiles (C) High-resolution(10mcellsize) shaded relief of Norfolk Canyon Features ( Obelcz et al. 2013).
Due to storm-driven re-suspension and a complex system of internal waves there is a variability in accumulation rates in the canyon. Along the slope the thickness of the sediments are a few meters thick but can vary between the different sample sites. Movement of sediments mostly occurs due to sediment spill-over near the shelf break, sediment creep on the upper slope, hemi pelagic
sedimentation, mass wasting and density flows within the canyon (Obelcz et al. 2013). Norfolk canyon
Distance into shelf (m) 16.5
Depth at shelf-edge (m) 800
Width at shelf edge (km) 8
Depth at canyon head (m) 80
Mean down-axis gradient (deg) 2.7
First axial bend (km) 7.5
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3. Methods
3.1. Field sampling
For this research five sediment samples from the Norfolk canyon were analyzed. These cores were collected during the research cruise RB13-03-HBH Deepwater Canyons Research Cruise 2013, with the NOAA vessel Ronald H Brown. Sediment samples were collected with a box corer designed by NIOZ, after retrieval of the box core, sub cores were taken by inserting a PVC liner in the sediment. The box cores which were analyzed for this bachelor thesis are indicated in figure 2 and table 2 with their exact position and water depth. The cores were stored at 8 °C for further analysis in the
laboratory at department for marine Geology and Chemical Oceanography at Royal NIOZ.
Figure 2. Multibeam bathymetric map created during the 2011 cruise (NF-11-02) shows the box cores taken in the Norfolk canyon, analyzed cores are indicated with a red frame, see Table 2 for exact positions.
Name Depth Latitude longitude Date
RB2013-036bx 1108 m 37.02.318 74.34.795 9 May 2013
RB2013-056bx 548 m 37° 00.948 74° 34.690 12 May 2013
RB2013-059bx 790 m 37° 00.542 74° 33.887 13 May 2013
RB2013-073bx 1106 m 37°00.346 74° 32.022 15 May 2013
RB2013-078bx 1622 m 37°02.008 74° 27.023 15 May 2013
Table 2 . exact locations of retrieved box cores in the Norfolk canyon during a cruise with the NOAA vessel Ronald H Brown in 2013 (Brooke and Ross 2014).
3.2. Core preparation
To open the cores they were placed in a core-cutter located at NIOZ and the PVC tube was cut lengthwise in two. With a knife the bottom and top lids of the cores were cut and the core itself was split with a wire. The half which was most representative was described with the use of a core description form. Sedimentological changes were noted as well as presence of carbonate debris, fragments of any kind, bore holes, mottling and colour of the sediment was described using a standard soil colour chart. One half was kept in the archive for further reference.
The other half was entirely used for sampling. First samples were taken every 5 cm core depth with the use of a syringe. These samples were labelled and stored in unipots. The rest of the core was sliced, whereby the first 5 cm was sampled every 0.5 cm and the rest of the core was sampled by
11 cutting each centimeter. When fragments, shells, rocks or corals where found they were labelled and taken out in order to make sure they didn’t get damaged by further procedures.
All the unipots were weighted empty without a lit and again when the unipots where filled with the sediment. All samples were put in a freezer of -20C° for at least two hours, then all the samples were placed in a Zirbus and Hetosicc freeze-dryer. Freeze-drying was used in order to extract all form of moisture from the samples without damaging the structure of the material itself. Due to the fact the samples had a lot volume, all samples were left in the freeze-dryer for more than 48 hours. After this procedure the dry weight was noted, these values were used in order to determine porosity,
moisture content and dry bulk density.
3.3. X-Ray Fluorescence-scanner and LineScan
In order to determine the presence of terrestrial and marine material the XFR scanner was used, the measurements are fast, non-destructive for the core and create a data file for the present elements measured in the surface sediment layer ( upper 3 mm) (Böning et al. 2007). Both parts of the core were placed in a Third generation Avaatech XRF Core Scanner combined with a line scanner which was used to create an image of the cores. The ratios between Ca/Fe and Ca/Ti were used as indication for the presence of terrestrial material.
Element ratios were measured with a run of 10kV and 30kV, this due to the fact that some elements are only detectable when a higher kV is used. With the use of X-ray radiation an electron is ejected from an inner shell of an atom with as result one of the electrons from an outer shell falls back. This causes an energy difference which is emitted as electromagnetic radiation and is unique for each element (Richter et al. 2007). With the use of an lead filter the sensitivity of the X-ray beam can be improved in order to measure certain elements. In order to prevent damage to the XRF scanner the surface of the core needs to be flattened and covered with a thin 4µm Ultralene film.
The cores were scanned with the following settings;
Tube voltage (kV) Filter Elements analysed Counting time Interval
10 None Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn 10 sec 10 mm
30 Pb thin Br, Rb, Sr, Zr 10 sec 10 mm
Table 3. Settings of the XRF-core scanner which were used in order to determine the required element ratios. 3.4. Grain size analysis
In order to study particle size distribution and sedimentation processes grain sizes were measured with the use of a Beckman Coulter LS 12 320 particle size analyzer. With the use of laser diffraction and light scattering, the particles scatter the laser light and generate unique angular scattering patterns. The Beckman Coulter LS 12 320 particle sizer can measure sizes between 0.04-2000 μm (Marum 2011). In order to have an optimal fraction of sample and laser light an obscuration between 10 and 20% is preferred. Obscuration is the intensity of the fraction un-scattered light, optimal values for obscuration are between 10 and 20 % when a run is 90 seconds with a pump speed of 53. Mostly optimal results are shown between 6 and 4 runs, the fraction of fine material can increase while measuring due to the fact small aggregates are destroyed when the sample is pumped around. Before the sample is measured the background value needs to be determined, this is due to the fact air can cause the laser light to scatter and therefore produce false values. For that reason Reversed Osmose water was used to dilute the samples ( Marum 2011).
From each core the upper 3 cm was measured at an interval of 0.5 cm, after the first 3 cm the interval was set at 2 cm. The freeze-dried samples have a weight between 0.1 and 5 g (de Stigter et
12 al. 2011) and were placed in a cup which was filled with approximately 40 ml of RO water and 10 ml of sodium pyrophosphate (0.1 M). In order to avoid flocculation of particles in the sample the prepared samples were stirred with the use of a magnet and an Ikamag Red until flocculation did not occur anymore. Particles above 2000 μm were removed with the use of a sieve in order not to damage the Beckman coulter laser (pers. Comm. J-B Stuut May 2015).
To achieve the preferred obscuration between 10 and 20% the sample can be diluted when the obscuration is to high > 20%. This was done manually by opening the drain and refill the sample holder with RO water, note that by losing a part of the sample often larger particles escape. When a sample has an obscuration which is too low this can be compensated by first discard part of the RO water in the system before adding the sample.
The obtained data from the preferred runs are exported with Beckman Coulter LS 13 320 software the metadata is used for grain size distribution.
3.5. 210Pb analysis
In order to obtain information of accumulation rates 210Pb activity was measured in the Norfolk
canyon sediments. 210Pb can be found in the oceans and presence of this type of Pb is a sign for recent
sediment accumulation. The accumulation rates of the last ~100 years can be obtained from sediment profiles of 210Pb. 210Pb is a radio-isotope with a half-life of 22.3 years (de Stigter, de Boer ,
Gieles 2011). 210Pb is produced in the decayseries of 238U, first 238U which is present in rocks is formed
into 222Rn which escapes as a gas into the atmosphere. Here the 222Rn decays trough intermediates to
the solid 210Pb and supplies the ocean in rain and dust form. Eventually 210 Pb is present in sea water
and will enter sediments on the ocean floor (de Stigter, de Boer, Gieles 2011).
Figure 3. Major sources of 210Pb and pathways to marine sediments (de Stigter, de Boer, Gieles 2011)
In recent deposited sediments located at the sediment-water interface an high activity of 210Pb can be
found with a decreasing trend down core. A background value will be reached deeper in the core which is a result of 210Pb from the already present 238U in the sediment. This background value is called
the supported 210Pb, the unsupported 210Pb is a product from the atmosphere and transported surface
sediment. The unsupported 210Pb can vary in space and time, thereby reworking of benthic organisms
can cause unusual fluctuations in 210Pb activity (de Stigter et al. 2011). By leaching the sediments the 209Po causes spontaneous deposition of 210Pb from the sediment on the silver discs, in order to prevent
iron to deposit on the discs ascorbic acid was added (Fernandez et al. 2012 & de Stigter et al. 2011). The deposited isotopes on the silver discs can be analyzed by a Canberra Alpha Analyst.
13 For each core 16 samples were analyzed, the first 3 cm was measured at an interval of 0.5 cm, after that an interval of 2 cm was chosen. In order to make sure the background value was reached it was essential to have three samples from the last few centimeters of the core.
First the freeze-dried sediment was grinded with the use of a mortar and pestle, in order to obtain an homogenous substance and enhance the leaching process. All samples were prepared in a digitube with a weight between 0.5000 and 0.5001 g of sediment, subsequently 1.0 ml of 209Po of tracer
solution was added to the sediment together with 10 ml hydrochloric acid (12m). The samples were leached on a temperature of 95℃ for 360 minutes. The second step in the leaching process was adding 5 ml of ascorbic acid with a concentration of 4% and 45 ml milli Q water, silver discs were placed in the digitubes and the solution was heated up to 80 ℃ for 960 minutes. After leaching the silver discs were taken out of the solution and cleaned with ethanol, the silver discs were dried before they could be analyzed with the Canberra Alpha Analyst.
After analyzing the differences in 210Pb deposition sediment accumulation rates were calculated with
the use of a model provided by Henko de Stigter (NIOZ). In order to calculate the actual accumulation rates ω (g cm-2 y-1) the supported 210Pb value and the initial activity were fitted in a one-dimensional,
two-layer diffusion model in which mixing only occurs in a surface mixed layer (SML). When using this model it is assumed that there is a constant 210Pb flux and sedimentation rate (de Stigter et al. 2011).
Figure 4. Formula to calculate the actual sediment accumulation rate, were ω is the sedimentation-rate (cm/y), λ is decay constant = 0.03114 (1/y) (λ = ln(2)/T½), were A0 and Az are the activity of excess 210Pb at
depth 0 and z, respectively (de Stigter, de Boer, Gieles 2011).
The sediment accumulation rate, initial supported 210Pb and diffusion rate were determined with the
solving function in Microsoft Excel on basis of the least squares best fit (Pers comm. de Stigter June 2015). Note that the cumulative mass depth (cmd) was used instead of a linear depth scale (de Stigter et al. 2011), this in order to avoid effects of different sediment compaction in the calculation.
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4. Results
In order to present and evaluate the retrieved data of the five cores taken in the Norfolk canyon an overview of all analyzed cores is given. Cores taken in the canyon and on the canyon open slope are compared to one another. Results will be described in trends on the shelf and within the canyon itself, the order of analyzing the data will be done by the water depth of each core which is in this case in a west to east direction. Thereby data obtained in earlier research will be used in order to create a complete overview of the processes in the Norfolk canyon.
Figure 5. Multibeam bathymetric map created during the 2011 cruise (NF-11-02). Cores NF12-0163, NF12-191, Rb13-036 and Rb13-078 are the cores taken in the canyon. The cores on the shelf are; NF12-182, Rb13-056, Rb13-059 and Rb13-073.
4.1. Sediment texture and composition
The five sediment cores taken in the Norfolk canyon are composed of greyish olive, olive black silty clay with some sandy silty clay and in core RB13-036 a clear sandy layer at 25 cm. In some cores fragments were found up to 1 cm and one coral fragment was found at the bottom of core RB13-056. Carbonate debris was present in most of the cores as single fragments up to 2 mm, bore holes were occasionally present in the upper 25 cm of the cores. Smell of decomposed organic matter occurred while opening the cores, additionally an oxidized layer of 0.5 cm was presented due to clear
difference in colour, brownish or greyish olive. A line scan taken of each core is shown in figure 6. Full core descriptions are available in the appendix.
15
Figure 6. Line scan created with the XRF core scanner located at NIOZ in April 2015. On the shelf
Core RB13-056 was taken at a depth of 548 m, located on a steep part of the open slope. This core consists of coarse sediment with fragments up to 3 cm which presented high rates of Fe when using XRF core scanner to determine the element ratios of these fragments. The texture is sandy silty clay with a olive black colour combined with fragments which start at the top with a size of 2 mm and become more coarse (up to 3 cm) deeper in the core. At a depth of 790 m on the canyon shelf core RB13-059 was taken which presents 1 cm of oxidized material with a dark olive colour, the rest of the core is olive grey and consists of silty clay sediment. The material becomes more coarse towards the bottom and some bore holes are visible between 8 and 10 cm from the top. When going towards the open slope core RB13-073 is taken at a depth of 1106 m. The texture of this core is also sandy silty clay with a olive grey colour, occasional carbonate debris was found.
In general the cores taken on the shelf show a significant difference in the presence of pebbles and grain size which is decreasing when going to deeper parts of the canyon.
16 In the canyon
In the canyon two cores were taken, one at 1108 m depth (RB13-036) and one at a depth of 1622 m (RB13-078). Both cores are located along the canyon axis which has a less steep slope compared to the open slope. The core at 1108 m depth and has a sandy layer at a depth of 25 cm to 27 cm, both cores are grey olive-olive black and sandy silty clay. Between 20 and 25 cm depth some bore holes and occasional carbonate debris were visible.
No pebbles and significant fragments were found in these cores, which is a main difference compared to the cores taken on the shelf.
4.2. Porosity
For each sample the porosity was determined and will be described in trends compared between the shelf and in the canyon. In order to create a complete overview data from earlier research on cores from the Norfolk Canyon is used as well to describe trends in the Norfolk canyon.
On the shelf
The mean porosity is higher in the two deeper cores, compared to the cores at 187 and 548 m depth, thereby all cores have a decreasing trend in porosity downward in the core itself. Average porosity fluctuates from 0.31% at 548 m depth to 0.59% at 790 m depth and 0.54% at 1106 m depth (figure 10). Except for the core taken at 187 m depth all trends show slight fluctuations in the core which differs from the upper 3 cm to the last 3 cm. No clear trend in relation to the depth of each core can be described here when looking at all the data obtained from the shelf.
17 In the canyon
In the canyon average porosity fluctuates from 0.58% at 559 m depth to 0.60% at 1108 m depth and 0.62% at a depth of 1622 m. The core at 1108 m depth contains a sandy layer, showing higher porosity up to 1.4% at 25 cm, which influences the average values. Besides that there is an overall trend when the depth is increasing on the canyon floor the average porosity is increasing
simultaneously. In each core there is also a significant decreasing trend with increasing depth (figure 8). All cores show a slight fluctuation in porosity in each core with as exception the core taken at 1108 m depth which is disturbed by the sandy layer at 25 cm depth which causes local a higher porosity.
For both the canyon and shelf the porosity is increasing with the depth of each core. When looking at the mean values between the shelf and in the canyon it can be stated that the porosity is higher in the canyon itself compared to the mean values on the shelf.
18 4.3. Grain size distribution
On the shelf
On the shelf of the Norfolk canyon the grain size distributions are visualized in figure 9, there is a trend of decreasing grain size with increasing water depth. In all cores there is a part of very fine material present with a size between 16.4 µm and 21 µm. The cores highest on the shelve have a distinctive peak at 168 µm which is very abundant in the first two cores but after that the cores only present very fine material with a size between 13 µm and 16 µm. All analyzed samples have a fraction of the very fine material and the more coarse grained material, there is no sudden shift in grain sizes with increasing depth in each core with as exception core RB13-036 which presents a sandy layer.
Figure 9. Grain size distribution for each core and their mean grain size, above the cores which occurs first on the shelf.
19 In the canyon
In the Norfolk canyon the grain sizes first show a decrease with depth in the canyon, except for the core taken at 1108 meter. This core presented a clear difference in grain size at 25 cm depth which disturbs the decreasing trend downward in the canyon (figure 9). Also in these cores there is a fraction of very fine material between 13 µm and 16 µm. The two first cores have a second peak at 168 µm which decreases towards the second core up to 140µm and increases significantly in the third core (1108 m) with grain sizes between 185 µm to 324 µm. This coarse part is only measured in samples between 25 and 27 cm in the core. Towards the end of the canyon these particles are not present anymore and only the very fine fraction remains in the cores.
Figure 10. Grain size distribution for each core and their mean grain size, above the cores which occurs first in the canyon.
20
Figure 11. Mean grain size and mean porosity for each core taken in the Norfolk canyon.
4.4. X-Ray Fluorescence results
The XRF core scanner provided data about the element ratios which were present in the cores. In order to determine the factor of present terrestrial and marine material the ratio between Ca/Fe and Ca/Ti was used. Even though Fe isn´t a stable element it is a good indicator for marine or terrestrial material. When both of these ratios are low it means that there is no indication for terrestrial material. The average value of the cores were taken and compared with the cores in the canyon and the cores on the shelf.
In the canyon
As is shown in figure 12 the average values of each core increases with depth along the canyon axis with as exception core RB13-078 which is located on the adjacent open slope. Furthermore core Rb13-036 shows some significant differences in Ca/Fe and Ca/Ti values when the depth of the core is increasing, this probably due to the presence of a sandy layer at 25 cm depth. This caused a higher value of Ca/Fe and Ca/Ti to occur.
On the canyon shelf
The average values for Ca/Fe and Ca/Ti in the cores taken on the shelf of the Norfolk canyon is different for each core. There is no clear trend visible on the shelf. When looking at figure 12 it can be stated that the average ratio on the shelf is higher compared to the ratios in the Norfolk canyon. With values of 1.53 for Ca/Fe on the shelf against 1.50 in the canyon and an average value of 8.78 for Ca/Ti on the shelf and 8.31 for Ca/Ti in the canyon.
21
Figure 12. Multibeam bathymetric map created during the 2011 cruise (NF-11-02). Ca/Ti and Ca/Fe ratios for each location in the Norfolk canyon.
4.5. Dating with 210Pb
The 210Pb activity shows a high variation in the cores, ranging from 729,9 mBq/g to 240 mBq/g in the upper centimeter of the cores. All cores show a clear decline in 210Pb activity deeper in the cores. But not all of the cores reached the background value of the 210Pb which is around 24 mBq/g. 210Pb activity in the deepest parts of the cores still range between 11,5 and 289,6 mBq/g. In core RB13-036 a layer of sand was found and shows a significant low value in the 210Pb profile and is left out when using the model. When the cores were compared between location, on the shelf and in the canyon itself there is a clear difference between activity. On average there is a higher accumulation rate in the canyon.
22 On the shelf
The cores located on the shelf of the canyon have in average a more smooth curve without a very disturbed profile. In all cores the background value is reached between a cumulative mass depth of 10 and 30 in the core. On the shelf a low activity of 210Pb visible except for the core RB13-059 which is located on a steeper part of the slope. The accumulation rates vary from 0.20 g cm-2 y-1 to 13 g cm-2 y-1. The first core has a significant higher value compared to the rest of the cores, after the first core there is an increasing trend visible in accumulation rates when the depth is increasing (figure 13). A surface mixed layer (SML) is present in the two first cores up to 4 cmd.
Figure 13. Sediment accumulation rates on the canyon shelf were ω is the accumulation rate, Db the diffuse biological mixing and Zmix the depth to which biological mixing occurs
23 In the canyon
The cores in the canyon itself are more active which is shown with the high 210Pb values and clear fluctuations within the core even when the depth is increasing. Thereby the background value is not reached in core RB13-036 or only at a cumulative mass depth between 20 and 30cm in the core. There is a significant difference in activity between the cores on the shelf and in the canyon, the cores deeper in the canyon have a higher activity. Except for RB13-078 which is located closer to the open slope of the canyon at a depth of 1622m. The sediment accumulation rates increase from 0.19 g cm-2 y-1 at a depth of 559m and increases up to 0.66 g cm-2 y-1 at a depth of 1108, after that the accumulation rate decreases again significant to 0.18 g cm-2 y-1 at 1622 m depth. A surface mixed layer (SML) is present in all the cores and varies from 1cmd up to 4cmd in the core (figure 14).
Figure 14. 210Pb accumulation profiles in the Norfolk canyon, were ω is the accumulation rate, Db the diffuse biological mixing and Zmix the depth to which biological mixing occurs.
24
5.
Discussion
When looking at the XRF data there is no significant trend in the canyon or on the shelf for Ca/Ti and Ca/Fe ratios, when looking at the mean values the ratios are slightly higher on the shelf compared to the values in the canyon. This slight difference in value can be caused due to the fact that marine sedimentation processes mostly occurs in the canyon itself and when the shelf is too active and has a low accumulation rate. Besides that there is a clear increase in Ca/Ti and Ca/Fe when the depth is increasing in each core.
In earlier research it was suggested that the Norfolk canyon used to be attached to the Susquehanna River-Chesapeake Bay drainage system (figure 15) which was active during the Pleistocene epoch (2,588,000 to 11,700 years ago) (Forde et al. 1979). Although this could have an influence in the Ca/Ti and Ca/Fe ratio it is not likely that the fluxes which are presented in the sediment cores are
influenced by this event. This due to the fact that the material stored in the cores are only presenting the last ~100 years. There is a possibility that there is more terrestrial material present deeper in the sediment layer which could been deposited in earlier stages. Other research suggested that material is transported from the Hudson river (Obelcz et al. 2013), in order to show significant trends and verify this age dating and more chemical analysis should be done (Brooke and Ross 2014).
Figure 15. Transport of fluvial material from the Chesapeake bay and James river to the Norfolk and Washington canyon (Eeescience.utoledo.edu, 2015)
The grain sizes decrease when the cores are located deeper in the canyon (figure 11), this is probably caused due to the fact fine material is transported at longer distances. One exception is core RB13-036 which is located at 1108 m depth and which shows a clear sandy layer at 25 cm depth in the core. This layer is not found in other cores, the origin therefore is hard to distinguish but could be formed due to a turbidite which occurs on very local scale (figure 16). This is important to keep in mind when looking at the porosity and 210Pb values of this core. Note that all data obtained from this core is highly fluctuating. On the shelf the grain sizes also show a clear decreasing trend deeper on the canyon shelf but is on average finer material compared to the grain sizes in the canyon. In core RB13-056 pebbles were found up to 3 cm, this can be due to the fact this core is located at the shelf edge and therefore erosion processes have more influence. Fine material is probably transported deeper in the canyon due to internal waves and mass wasting.
25
Figure 16. Turbidity in the Norfolk canyon (Brooke. and Ross. 2014).
When looking at the accumulation rates in the canyon there is a strong increasing trend up to a depth of 1108 m. This is probably due to the fact material is not only transported from shallow to deep parts but also by currents which are coming in from the north and the south of the canyon. The first three cores therefore have three sources which can provide sediment, core RB13-036 has the highest accumulation rate of 0.22 g cm-2 y-1. This is probably due to its location between the shelf
edges were internal waves are formed and therefore transport more material. When going to deeper parts of the canyon the accumulation rate decreases again due to de lack of internal wave activity and as a result less sediment which is re-suspended.
Figure 17. Multibathymetric map of the Norfolk canyon, indicated are the depth of each core and the accumulation rate ω.
26 On the shelf there is an increasing trend from a depth of 548 m, this is due to the fact core
RB13-056 is located at the shelf edge and is strongly influenced by internal waves which causes re-suspension from this location and therefore a slower accumulation rate is visible. The sediment is transported deeper into the canyon and therefore causes an higher accumulation rate at deeper locations. The angle of the slope is an important factor for accumulation rate, when a core is located on a steep and more rough shaped slope material will re-suspend by mass wasting or by internal waves (Obelcz et al. 2013). When looking at figure 16 it is visible that there is more active turbidity between 500 and 800 meter depth (Brooke and Ross 2014).
In the upper centimeters of the core benthic reworking occurs, this causes variation in 210Pb activity and form irregularities in an otherwise regular profile (Obelcz et al. 2013). Diffusive mixing therefore is an important variable when calculating the actual accumulation rates and can occur up to 20 cm in the cores. Other important factors which play an important role in sediment accumulation rates are spill-over and internal waves. Turbidity is active but does not cause significant fluctuations, in the Baltimore canyon for example turbidity is very active and therefore plays an important role in resuspension of sediment. Topography of the highly irregular canyon floor together with internal waves due to currents causes significant differences in accumulation rates between the cores and should be taken into account when looking at any type of data from the Norfolk canyon.
The research done for this bachelor thesis is only a small part of analyzing sediments from the Norfolk canyon. In order to determine the origin from the sediment and understand element cycling in the Norfolk canyon more chemical analysis should be done. When looking at the obtained data it should be taken into account that all processes occur at a very local scale and therefore results can be significant different when other cores form the Norfolk canyon are analyzed. In addition it should be taken into account that external factors such as tracer liquids and used chemicals can influence the preformed analysis. When analysis is repeated these external factors can cause fluctuations in the obtained data.
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6. Conclusion
The research questions stated in the introduction were;
“What are the sediment accumulation rates in the five different cores taken in the Norfolk canyon.”
“What are the particle transport processes in the Norfolk canyon which causes the difference in accumulation rates.”
“How can distinctive zones within the Norfolk canyon be described and what are the main processes which causes this difference .”
In order to answer these questions data was obtained at the Royal NIOZ which provides more insight in the processes which are active in the Norfolk canyon. Two distinctive zones were determined in the Norfolk canyon and used to analyze the data, namely there is a distinct difference between the cores in the canyon and on the canyon shelf. When looking at the data it can be stated that location of the core is one of the most important aspects which is related to grain size distribution, porosity and accumulation rates. The interaction between topography of the canyon and the canyon floor, canyon morphology and hydrography causes sediment to re-suspend or accumulate. Thereby the particle transport processes are depended on the topography and currents which are coming in from the north and south of the canyon. Due to these currents internal wavers are generated along the edges of the canyon which causes material to re-suspend and can increase potential turbidity. Mass wasting occurs mostly on steep and instable slopes and can be triggered by the internal waves. When looking at the sediment cores there is a clear sign of bioturbation which is caused by benthic activity which can occur up to a depth of 20 cm in the cores and is visible in the high fluctuating 210Pb activity and the disturbed decreasing trend downwards.
Sediment accumulation rates are thus strongly influenced by the specific location of each core, the topography of the canyon floor and present currents and internal waves. For the Norfolk canyon there is a significant difference between the activity in the canyon and on the shelf due to these external factors. These processes causes that every submarine canyon is unique and will show different patterns in accumulation rates and main sources of sediment.
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7. Evaluation and acknowledgement
First of all I’m very pleased by the fact I had the opportunity to do this research at the Royal NIOZ, it gave me a better insight in the real world of science which I’m very interested in. When I came to the NIOZ it was very clear what type of research and analysis needed to be done. Every step of the process was shown to me once and after that I had to work independent. It was my responsibility to obtain all the needed data before the 12th of June, time management was very important in order to finish all analysis on time. The fact my supervisor trusted me to work individually and gave me responsibility was definitely one of the main things I enjoyed during my internship at NIOZ. Besides that there was always the possibility to ask someone for help or advise, I learned to ask for help when needed. Due to the fact I scheduled most of the work myself and was responsible for the outcome of this research caused that I was more connected to my work and processes. In order to work individually I needed to understand each step of the process, this increased my motivation to understand, question and read information about this subject.
The importance to work precise and accurate was something I realized and improved while being here, mainly due to the repetitive work I realized it can cause inaccuracy when you are not focused. In addition to that the fact that the data I obtained could be used for further research affected my I awareness of possible mistakes or inaccuracy.
Due to the fact I became more connected to the subject I discovered that there is always something new coming up when doing research. When more time was available I would be able to perform different types of analysis. Bottlenecks in relation to timing did not occur during this thesis which definitely surprised me due to the fact I depended on different types of machines which can break down or produce false data. After this internship I realize that I’m capable of working individual and that I take responsibility when necessary. Due to the fact I had time between my different analysis I was able to write a large part of my thesis while doing my internship at NIOZ. This caused that writing my thesis was not a huge task after all the data was obtained and as result the writing went fluent. I’m aware of the fact this internship wouldn’t have been an success without guidance of my
supervisor Dr. Furu Mienis who I want to thank for the opportunity, her kindness and trust in me. Also I want to thank Piet van Gaever, Rieneke Gieles and Jan-Berend Stuut who helped me in the lab during different analysis. Henko de Stigter, who explained the 210Pb models to me which gave me my actual data about accumulation rates. Further I want to thank everybody else who helped and supported me during my internship at NIOZ.
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