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Geophysical Research Letters
Supporting Information for
4 An abyssal hill fractionates organic and inorganic matter in deep-sea surface sediments
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Robert Turnewitsch 1 , Niko Lahajnar 2 , Matthias Haeckel 3 , and Bernd Christiansen
1 Scottish Association for Marine Science (SAMS), Oban PA37 1QA, United Kingdom
2 University of Hamburg, Institute for Geology, Bundesstraße 55, D-20146 Hamburg, Germany
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3 GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3, D-24148 Kiel, Germany
4 University of Hamburg, Institute for Hydrobiology und Fisheries Sciences, Große Elbstraße 133, D-22767 Hamburg, Germany
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Contents of this file:
Text S1, including Figures S1 to S9 and Table S1.
Additional Supporting Information (Files uploaded separately)
Not applicable.
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21 Introduction
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The Supporting Information (SI) provides additional detail on (a) the wider environmental setting of the study region, (b) aspects of specific, relevant environmental conditions during the study, and
(c) analytical methods used. In addition, figures are included that identify each individual sediment core and sampling station (whereas, for clarity, the corresponding figures in the main text only distinguish between hill and Far-Field stations).
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27 1. Further details on sampling and analytical methods
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The samples for this study were collected during cruise 260 of RV Poseidon (26 April - 23
June 2000). The study region is located in the temperate Northeast Atlantic on the Porcupine
Abyssal Plain (PAP). One sediment core was sampled with a normal multiple corer, all other cores were collected with a small version of a multiple corer (‘mini multiple corer’). All sediments from the hill were collected with the mini corer. Both corers were equipped with core liners of 10 cm inner diameter. Further sediments from the abyssal plain were retrieved (subsampled) from incubation chambers mounted on free-falling benthic-lander systems. Sampling types and locations are shown in Fig. S1.
In the Far Field, six sediment cores from different sites were analysed for AAs and HAs, and twelve for PIC, BSi, OC, N and LM. These core numbers allow us to gain some insight into spatial variability of each parameter in the Far Field on length scales of decimetres up to hundreds of meters which, in turn, facilitates the search for differences between the Far-Field and hill sites. The PIC uncertainties (precision) in Fig. 2d are given as one standard deviation.
This information on small-scale variability is crucial as, for the hill sites, often only one core could be collected and analysed per site. At Far Field SW, the core numbers per parameter are as follows: PIC, OC, N: 2 cores; AA, HA: 1 core. On the hill, the core numbers per parameter were as follows. North Slope and East Slope: PIC, OC, N, AA, HA: 1 core; West Slope and
Summit: PIC, OC, PN: 2 cores; AA, HA: 1 core. For PIC on the West Slope and the Summit, uncertainties in Fig. 2d are given as the range between two values. For PIC on the North and
East Slopes, no uncertainties could be given. For the calculation of uncertainties in Fig. 2a-c, see Turnewitsch et al. (2004,2013) for details. The methods used to determine grain-size distributions and 210 Pb activities were described by Turnewitsch et al. (2004).
There were two sets of samples, (1) and (2), in which sedimentary OC and N contents were determined in slightly different ways. Set (1) comprises one core from each of the following stations: 94#2, 100, 113#2, 118, 148 and 153(1); set (2) contains all the other cores (see
Fig. S6 for all individual core profiles being identified).
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In set (1), sediment pore-waters were removed by pore-water pressing and the remaining sediments were freeze-dried and ground. Subsamples were analysed for total particulate carbon (TPC = POC + PIC) and total particulate nitrogen (TPN) by flash combustion in a Carlo
Erba Element Analyzer (NA 1500). In companion subsamples, PIC was driven out of the sample with HCl prior to POC analysis. PIC was obtained by subtracting POC from TPC.
Relative analytical accuracy and precision of the method are 5 % and 2 %, respectively. In sample set (2), pore waters were not removed prior to analyses. Total carbon (TC), total organic carbon (TOC) and total nitrogen (TN) were analysed with a Carlo Erba NA-1500
Elemental Analyzer following the analytical methods described by Verardo et al. (1990) and
Nieuwenhuize et al. (1994). PIC was calculated by subtraction of TOC from TC. The fact that the POC (PN) data from sample set (1) do not capture DOC (DON) is not relevant for this study as DOC measurements in selected Far-Field samples have shown that DOC only constitutes
~23‰ of TOC. The POC (PN) and TOC (TN) data from sample sets (1) and (2), respectively, were therefore combined in Fig. 3 under the label OC (N).
In sample set (2), Biogenic opal (BSi) was measured according to a slightly modified method after Mortlock and Froelich (1989). The lithogenic fraction was calculated as the remaining part of a sample after subtracting the carbonate, organic matter (TOC x 1.8) and BSi fraction. Amino acids (AA) and amino sugars (HA) were analysed with a Pharmacia LKB Alpha Plus 4151
Amino Acid Analyser. In brief, sediment samples were hydrolysed with 6 N HCl for 22 hours at
110°C. After hydrolysation samples were evaporated and taken up in an acidic buffer (pH 2.2), separated by a cation exchange resin and detected fluorometrically after post-column derivatization (see Lahajnar et al. (2007) for details).
Sediment sampling was carried out during the arrival of a settling plankton bloom in the near-seafloor water column. Potential effects of this arrival on the interpretation of the sediment data are discussed in Section 3 of the SI. It will be concluded that the hill-versus-Far-Field differences in the organic-matter constituents are in fact a spatial (hill-induced) rather than a temporal (bloom-related) phenomenon.
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82 2. Additional environmental information
83 2.1 Fluid dynamics around the Porcupine Abyssal Plain hill
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The approaches to numerically model the interactions of background flow and tides with the topography were described by Turnewitsch et al. (2004,2013). As outlined in the main text of this paper, the reported distributions of PIC and organic-matter constituents are thought to be ultimately due to the interactions of the different components of the ocean flow with the topographic elevation. Here, we provide further information on the flow patterns and on measures of the vigorousness of the near-seafloor fluid dynamics. The aim is to identify the main flow features and to define the flow indicator I at h
The case-study hill is located on the northern hemisphere at a midthe Porcupine Abyssal Plain (PAP) in the Northeast Atlantic. The hill is topographically relatively isolated and ~ 2 times wider in the NW-SE direction (width at the base: D ≈ 16 km) than in the NE-SW direction (width at the base: D ≈ 7 km) (Fig. S1). The main summit is at h m
≈
900 m above the surrounding plain and the plain is typically at a water depth of H ≈ 4850 m.
There is a secondary summit further to the southeast, with a saddle between the two summits
≈ 600 m above the surrounding abyssal plain. The southwestern slopes are much steeper than the northeastern slopes (Fig. S1).
To the south of the hill, Vangriesheim et al. (2001) conducted a highly resolved time-series study of current velocities at 15 meters above bottom (mab). It turned out the typical background (residual) flow is quasi-steady on time scales of weeks to months and impinges on the hill from southerly directions (for 50, 37 and 13% of the time, background currents came from southerly, southwesterly and eastsoutheasterly directions, respectively). The average current speed of the quasi-steady background flow
F
that is used in Fig. 2.
U sf latitude of 49.12°N on
(i.e., the flow without tidal and other higher-frequency current-velocity fluctuations) was determined to be 2.8 cm s -1 . Due to conservation of potential vorticity, the interaction of the quasi-steady background flow
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108 component with the hill leads to formation of a flow-field asymmetry around the hill (Fig. S2)
(Turnewitsch et al., 2004,2013).
Fig. S1. Sampling sites on and near the
Porcupine Abyssal
Plain (PAP) hill, superimposed on a map of maximum total tidal current speeds
( u tmax
). The featureless region in the northwestern corner of the map does not contain any data. Total tidal current speeds contain flow components of the barotropic and baroclinic tide and also of ‘trapped waves’ that result from the interaction of the diurnal barotropic tide with the hill. Results are derived from a numerical tide model that was run for 62 days (Turnewitsch et al., 2013). All sediment, sedimenttrap and photographic sampling sites of this study are indicated.
Isobaths are shown at
50 m-intervals, with the surrounding abyssal plain at typically 4800 –4850m depth.
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Vangriesheim et al. (2001) showed that there are very clear current-speed and currentdirection oscillations at or near the semidiurnal frequency ( ω ts
= 0.00014 s -1 ), superimposed on the background flow. There was also evidence for near-inertial oscillations. However, the energy contained in semidiurnal tidal currents was around 5 times that of the near-inertial oscillations (Turnewitsch et al., 2013). Model-derived estimates (Egbert and Erofeeva, 2002) for typical barotropic semidiurnal and diurnal (frequency ω td
= 0.00007 s -1 ) tidal current-velocity amplitudes u
1 s
and u
1 d
are 1.68 – 5.24 cm s -1 and 0.179 - 0.815 cm s -1 , respectively; total barotropic tidal current-velocity amplitudes u
1 t
are 1.24 – 5.63 cm s -1 . In the far-field, the major axis of the total barotropic tidal current-velocity ellipse runs close to the NW-SE direction. The shape and dimensions of the far-field rotations/oscillations of the total barotropic tidal current velocity are strongly dominated by the semidiurnal tidal constituents.
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Fig. S2. Topographically controlled sediment-sampling sites, superimposed on modelled quasisteady background flow near the seafloor on and around the PAP hill for far-field inflow from the south at a Rossby number of Ro s
= U sf
/ ( f
L
G
) ≈ 0.03 ( U sf
: current velocity of quasi-steady background flow in the
Far Field; f : Coriolis parameter; L
G
: measure of the width of a topographic elevation). Long vertical arrow in the upper right corner: indicates 4 times far-field inflow speed. Isobaths are plotted at
100 m intervals (most central isobath: 4000 m; outermost isobath:
4700 m).
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The water column around the hill is weakly mass-density-stratified and has a buoyancy frequency of N ≈ 0.0002 – 0.0004 s -1 (WOCE climatology: Gouretski and Koltermann, 2004). If tidal flow encounters a topographic obstacle in mass-density-stratified waters, internal
(baroclinic) tides may form. If this internal-tide generation occurs at geographical latitudes at which the tidal frequency is higher than the inertial (Coriolis) frequency, the internal tides remain trapped at the topography; otherwise, they can radiate away. At the PAP hill, diurnal internal tides would be trapped whereas semidiurnal internal tides would radiate away.
Figure S1 shows the numerically modelled distribution of maximum total tidal current speeds on and near the studied hill that result from the combination of total barotropic and semidiurnal and diurnal baroclinic flow components.
We think that maximum (rather than average) current speeds constitute the most important parameter for deposition of freshly introduced particulate matter at the seafloor. At a given location, maximum speeds result from the temporally varying combination of the different flow components (here, mainly background and tidal). For the sediment-sampling sites of this study, such maximum speeds were estimated based on the aforementioned fluid-dynamics modelling of the tidal (Fig. S1) and background (Fig. S2) flow fields around the hill (Turnewitsch et al.,
2013). In this study, because of the evidence for variations in the inflow direction of the background flow on time scales of several months (Vangriesheim et al., 2001), three different inflow directions of the background flow were considered: flow towards the northwest, north and northeast. In all cases the far-field current speed of the background flow was assumed to be constant at 2.8 cm s -1 . The topographically controlled background current velocities were vector-added to their respective total (barotropic + baroclinic + trapped-wave) tidal current velocity components. This was done for a representative 62-days tidal time series. The resulting maximum total current speeds are shown in Fig. 2a for each main sediment sampling site and the three prevailing inflow directions of the background current.
It is commonly assumed that, for a given sediment, current speed is the key determining factor for sediment (non-)deposition, erosion and resuspension. There is, however, evidence
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230 for higher-frequency (tidal and/or (near-)inertial) variability of current direction to also play a role. To take the directional effect into account as well, a new measure was introduced that combines information on both current speed and the intensity of directional variability
(Turnewitsch et al., 2013). First, a measure of the intensity of higher-frequency temporal variability of current direction ( S sd
) was defined. To estimate S sd
, the directional 360° circle (with
0° = 360° pointing northwards) was split into 8 evenly spaced sectors, starting at 0°. Then, for each semidiurnal tidal cycle, the number of observed directional sectors was determined and this number was divided by the length of the cycle (12 hours). Second, to investigate the possible combined influence of current-direction variability and current speed on sediment dynamics, another non-dimensional indicator, I
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, was defined that is composed of V max
and S sd
:
I
F
= ( V max
/ V c
) ( S sd
/ S max
), with V max
and V c
in the unit of m hr -1 and S sd
and S max
in the unit of sectors hr -1 , and where V c
≡ 0.07 m s -1 is the critical near-seafloor current speed for resuspension of freshly deposited phytodetritus (Lampitt, 1985; Beaulieu, 2002) and S max
≡ 0.667 is the maximum possible value of S sd
. I
F
was calculated for each semidiurnal modelled time interval of each far-field inflow scenario at each main sediment sampling site. For each whole time series a temporal mean, standard deviation and median were calculated. Then, for each sediment sampling site and quasi-steady inflow direction, a spatial mean and standard deviation of the aforementioned temporal medians were calculated.
Maxima of the spatiotemporally averaged I
F
values are arranged as follows: Summit >
West Slope > East S lope ≈ Far Field (abyssal plain) > North Slope (Fig. 2a). Maximum I
F values are very strongly and negatively related to the intensity of sediment focussing, as reflected in the average ratio of expected (water-column-derived) and measured (sedimentderived) fluxes of the naturally occurring radioactive particle tracer 210 Pb (Fig. 2a,b). Because of its half-life of 22.3yr, 210 Pb data capture sedimentary processes that occurred over the past few decades and reflect current and recent fluid dynamics. There is also an influence of the hillcontrolled near-seafloor fluid dynamics on the grain-size distribution in surface sediments: here,
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243 maximum current speed seems to be the controlling factor (Fig. 2a,c) (Turnewitsch,
2004,2013).
It can be concluded that the interactions of the hill with the ambient ocean flow leave an imprint in the underlying sediments (Turnewitsch et al, 2013), with sediment grain-size distributions being controlled by maximum current speeds and sediment focussing and winnowing being controlled by the combined influence of maximum current speeds and higherfrequency (tidal) directional current variability.
2.2 Seafloor photographs
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In a few locations on the upper northeastern slope of the hill, seafloor photographs could be obtained with a Deep-Sea Observation System (DOS). The DOS consisted of a Benthos
Standard camera, flash and altimeter. The camera contained a Kodak Ektachrome 100 film.
When photos were taken, the distance to the seafloor was ~ 3 mab. During the photographic surveys the ship drifted at a speed of ~ 1 knot towards northeasterly directions. Photos were taken every 15 seconds. However, due to problems with the altimeter and also the camera, the number of quality seafloor photographs was severely limited.
Two deployments were made, yielding a total of 39 useful photographs from an area just east-northeast of the main summit (deployment DOS 10/11: station 136 at a nominal location of
49.1198°N, 16.6345°W at a nominal depth of ~ 3900 m; DOS 12: station 149 at a nominal location of 49.1161°N, 16.6333°W and a nominal water depth of ~ 3900 m) (Fig. S1). These photographs reveal a very varied terrain, ranging from areas with a very thin sediment cover
(Fig. S3a) over rocky outcrop areas where hardly any soft sediment could be seen (Fig. S3c) to flat areas with virtually continuous sediment cover Fig. S3b,d-h). The latter differed in the amount and sizes of rock debris. The examples of seafloor shown in Fig. S3g,h are most representative for this area of the hill, i.e., a virtually continuous soft-sediment cover interspersed with lower numbers of smaller pieces of rock debris. The sediment surface was structured with numerous animal tracks and traces (in other photos, in terms of megafauna, the
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268 holothurian Amperima, xenophyophores, actiniarians and stalked crinoids (attached to rocks) could be seen) . There is some evidence for ‘lumps’ of phytodetritus that accumulated in microtopographic depressions of the sediment surface. The shape and dimensions of the cloud of artificially resuspended sediment in Fig. S3f suggest the deposited sediments contain finegrained material. The photos indicate that any amounts of freshly deposited phytodetrital material must have been small in this area of the hill at the time of the photographic survey
(06/07 June 2000).
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Fig. S3. Seafloor photographs from DOS deployment at station 136 on the uppermost part of the eastern shoulder of the main summit at ~ 4000 m (see Fig. S1 for location). Distance to the seafloor: approx. 3 m.
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273 3. The arrival of a settling plankton bloom
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To our knowledge, the first amino-acid (AA) data for sediments in the PAP region were reported by Horsfall and Wolff (1997). For surface sediments that were collected between 12
May and 3 June 1991 on the far-field abyssal plain (very close to the deployment site of our sediment-traps mooring: Fig. S1), they found total-AA concentrations and percentages of AAcontained carbon in total organic carbon (TOC) that were higher than in our Far-Field sites. The samples of Horsfall and Wolff (1997) and our samples were collected during a very similar time of the year. But the timing of the arrival and the flux magnitude of the seettling bloom material in the deep water column above the PAP region are known to show intense interannual variability
(Lampitt et al., 2010; Salter et al., 2010). It could therefore be that, in 1991, the settling bloom material arrived at the seafloor much (weeks) earlier than in 2000, even allowing the fresh and semi-labile material to be (partly) bioturbated into the surface sediments before the samples were collected. The potential influence of temporal variability therefore needs to be considered when it comes to the interpretation of our results.
The potential importance of temporal aspects becomes even more relevant as sediment sampling for our study was carried out shortly before and during the arrival of a settling plankton bloom (Fig. S4). It, therefore, needs to be established whether the sedimentary differences we find between the hill and the Far Field, as reported in the main text, are in fact spatial ones rather than primarily temporal ones that resulted from the fresh particles arriving and leaving an imprint on the upper hill before they did so on the lower hill and in the Far Field.
Using a combination of information on the appearance and chemical composition of particles and on the timing of sediment sampling in different locations relative to the timing of the arrival of the settling bloom particles, we are able to show that the sediment-biogeochemical differences between the hill and the Far Field are in fact spatial (hill-controlled) and not temporal (bloom-controlled) ones.
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Fig. S4. (Top) Current speeds of the barotropic tide in the Far Field (time series extracted from the inverse tide model of Egbert and Erofeeva, 2002). Short vertical lines at the top: boundaries between trap-sampling intervals. (Bottom) Time series of downward fluxes of total particulate matter (TPM) and particulate organic carbon (POC) at 569 mab above the abyssal plain in the Far Field to the south (upstream) of the PAP hill (see Fig. S1 for exact location).
Horizontal lines indicate the sampling intervals of the sediment trap. Times of sediment (MUC, mMUC, FFR) and photographic (DOS) sampling on and off the hill are indicated by vertical lines.
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Detection of the arrival of the settling bloom particles ---- The arrival of the settling bloom particles was detected by three sediment traps that were deployed at 2 mab (‘Kiel-type’ trap on a benthic lander system) and at 40 and 569 mab (Mark 7 traps on a mooring) on the upstream side (to the south) of the PAP hill. The benthic lander (FFB-01) was deployed at station 65#1 (4802 m; 48.8315
°N, 16.5300°W) and the mooring at nearby station 64#1&2
(4808 m; 48.8265°N, 16.5018°W) (Fig. S1). The sampling interval per trap cup was 7 days and the mooring traps operated for 7 intervals (01/05/2000 01:00 UTC – 19/06/2000 01:00 UTC) whereas the lander trap collected for 6 intervals (08/05/2000 01:00 UTC – 19/06/2000 01:00
UTC).
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Macroscopic appearance of particles ---- Cursory macroscopic examination of the sediment-trap samples from 40 and 569 mab revealed that collected particles consisted “almost exclusively” of “green-brown” phytodetritus (Lahajnar and Schroll, 2001). At 569 mab the relative macroscopic contributions of large amorphous aggregates (> 1 mm), small amorphous aggregates, coarse-grained material, fine-grained material and faecal pellets < 1 mm were
‘trace’ (< 10%), major (50-100%), trace, minor (10-50%) and trace, respectively. The samples from 40 mab looked similar except for a complete lack of large amorphous aggregates during the first two weeks of the deployment period.
The macroscopic appearance of the samples from 2 mab differed more clearly from the appearances of the samples from 40 and 569 mab. The material also almost exclusively consisted of phytodetritus, but looked “yellow-brown” rather than “green-brown”. At 2 mab, the major contributor to the trap material throughout the deployment period was fine-grained material. Only during the last 1 or 2 weeks of the deployment period, there were trace amounts of large amorphous aggregates. Throughout the deployment period there were only minor amounts of small amorphous aggregates, and only trace amounts of coarse-grained material occurred during only two deployment weeks. Overall, the material at 2 mab looked “less mucilaginous” than at the other two sampling heights above the seafloor.
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Foraminifera occurred only in trace amounts in all trap samples at all three heights above the seafloor throughout the deployment period. No faecal pellets > 1 mm were observed in any of the trap samples (Lahajnar and Schroll, 2001).
At 2 mab, the different particle colour, the almost complete absence of large amorphous aggregates and the shift in predominance from small amorphous aggregates to more compact fine-grained material strongly suggest a major influence of benthic-boundary-layer processes on particle properties and probably the nature of particle deposition and sediment formation.
Because of the very clear influence of the benthic boundary layer at 2 mab and the likely influence of the variable upper bottom-mixed-layer boundary (Turnewitsch et al., 2001) on the trap fluxes at 40 mab, we will focus the following discussion on the particles that were collected at 569 mab.
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Temporal compositional trends of settling particulate matter at 569 mab plankton bloom in the sunlit layers of the surface ocean.
---- Net downward fluxes of total particulate matter (TPM) and particulate organic carbon (POC)
(Fig. S4,S5a) as well as of particulate nitrogen (PN) (Fig. S5a), amino acids (AA) and hexosamines (HA) (Fig. S5b) increased throughout the sampling period, all illustrating the arrival of increasing amounts of fresh, organic-rich material that must have originated from a
The sedimentary parameters that showed hill-vs.-Far-Field differences are the weightpercentages of OC and N in TPM, weight percentages of AA carbon (AA_C) and HA carbon
(HA_C) in TOC (AA_C / OC and HA_C / OC), molar AA / HA, Gluam / Galam, Asp /

-Ala,
Glu /

-Aba and B / (B + A) ratios, the AA degradation index DI, and the molar fraction of the sulphur-containing AA methionine (Met). How did these parameters change over time in the settling particles that were collected at 569 mab?
Weight-percentages of POC and PN in TPM increased (Fig. S5c) as did AA_C / OC and
HA_C / OC (Fig. S5d), all also illustrating the arrival of fresher, organic-rich material. In relative terms, AA_C / OC and HA_C / OC changed in similar ways, resulting in hardly any temporal
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384 change in AA / HA, the only exception being the last week of the sampling period when a slight drop in AA / HA coincided with the highest TPM and POC fluxes (Fig. S4 and S5d).
Here, HAs are defined as the sum of the aminosugars galactosamine (Galam) and glucosamine (Gluam). Gluam is known to occur in chitin (mainly crustacean exoskeletons and peritrophic membranes of their faecal pellets, but also mollusc radulae and beaks and shell interiors of cephalopods). Gluam has also long been known to be a component of the cell walls of many prokaryotes (most bacteria, some archaea) but more recent work has shown that the majority of Gluam in prokaryotes is not associated with the cell-wall polymers but seems to occur in other (more easily degradable?) parts of the cells (Benner and Kaiser, 2003). Galam occurs in prokaryote cell walls but only in trace amounts in zooplankton (Müller et al., 1986).
Except for certain types of cyanobacterial Synechococcus and the diatom Thalassiosira oceanica , the molar Gluam / Galam ratios for photo- and heterotrophic bacteria and phototrophic algae are on the order of 1; for copepods (chitinaceous exoskeletons), they are on the order of 10 (Benner and Kaiser, 2003). Therefore, when it comes to bulk particulate organic matter (POM), in most cases, Gluam/Galam ratios > 3-4 are likely, and ratios > 8 virtually certain, to indicate large amounts of zooplanktonic (chitinaceous) material. By contrast, lower ratios would indicate predominantly prokaryotic and/or phytoplanktonic organic matter. As the majority of Gluam is not associated with prokaryotic cell-wall polymers, very low Gluam / Galam values may not only indicate almost complete absence of chitinaceous material but probably also advanced decomposition of hexosamines and hexosamine-containing biopolymers, and, in extreme cases, even suggest a drop of active prokaryote biomass.
In the settling material at 569 mab, Gluam / Galam ratios showed an increase from ~ 2-3 to
~ 4-5 during the deployment period (Fig. S5d). Interestingly, the increase of Gluam / Galam ratios is due to increasing Gluam rather than decreasing Galam, the latter remaining virtually constant throughout the deployment period. The Gluam increase also indicates fresher and more easily degradable (bioavailable) organic matter. As the increase reaches above ~ 3 it could also suggest an enhanced zooplanktonic (chitinaceous) component in the settling
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397 material. Given the macroscopic appearance of the bulk material, as described above, the chitinaceous material must have consisted of carapace remnants and/or peritrophic membranes of very small faecal pellets of crustaceans rather than mainly intact carapaces.
However, the very weak and late decrease in AA / HA ratios supports the notion that the relative zooplankton influence on total organic-matter fluxes remained small (Haake et al.,
1993).
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Fig. S5. Sediment-trap time series of organic-matter-related parameters of the settling material at 569 m above the abyssal plain in the Far Field south (upstream) of the PAP hill. (a)
Downward fluxes of particulate organic carbon (POC) and particulate nitrogen (PN). (b)
Downward fluxes of total amino acids (AA) and hexosamines (HA). (c) Weight-% in total particulate matter (TPM) of total organic carbon (TOC) and total nitrogen (TN). (d) Amino-acid carbon (AA_C) and hexosamine carbon (HA_C) as weight percentages of POC, molar AA / HA ratios, and molar ratios of the HAs glucosamine (Gluam) and galactosamine (Galam). (e) Molar ratio of the protein-AA aspartic acid (Asp) and non-protein-AA

-alanine (

-Ala), molar ratio of the protein-AA glutamic acid (Glu) and non-protein-AA and

-aminobutyric acid (

-Aba), and the
AA degradation index DI. (f) Molar ratio B / (B + A) of basic AAs (positively charged side chains; B = histidine + arginine + lysine) and the sum (B + A) of basic and acidic AAs (A = Asp
+ Glu; negatively charged side chains); and mol-percentage in all AAs of the sulphur-containing
AA methionine (Met). Horizontal lines indicate the sampling intervals of the sediment trap (for clarity, they are shown for only one parameter per diagram).
18

413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
In addition to total AAs, individual AAs were also analysed in the settling particles. Both molar Asp /

-Ala and Glu /

-Aba (Fig. S5e) increased during the last two weeks of the deployment period. As the non-protein AAs

-Ala and

-Aba are viewed as degradational products of the protein AAs Asp and Glu, respectively (Degens et al., 1963; Lee and Cronin,
1982), the increasing ratios also indicate the arrival of fresh material. Furthermore, in absolute terms, the AA degradation index DI (sensu Dauwe et al., 1998,1999) displayed little variability over time (-0.21 – 0.13), with the highest (most positive) value occurring during the last week of the deployment period when the highest mass flux was determined at 569 mab (Fig. S5e).
Given the reported range of DI in settling particles (Keil et al., 2000: ~-1 up to ~2), the measured temporal variability of DI at 569 mab in this study is very small. However, the DI values in the sediment-trap material at 569 mab are consistently higher than the DI in the surface sediments both on the hill and in the Far Field. Finally, t he fresh ‘high-flux’ material during the last two weeks of the deployment period was also enriched in acidic AAs (A) (relative to basic AAs (B), leading to reduced B / (B + A) ratios) and in the sulphur-containing AA methionine (Met) (Fig. S5f).
433
434
435
436
429
430
431
432
437
438
439
Comparison of temporal trends at 569 mab with sedimentary hill-vs.-Far-Field differences ---- As mentioned at the end of Section 1 above, it could be argued that the observed sedimentary differences between the hill and the Far Field could be temporal rather than spatial ones. The hill sites are closer to the sea surface than the Far-Field sites and bloom material settling from the surface ocean would reach the hill sites before it reaches the deeper
Far-Field seafloor. Can the temporal trends in biochemical composition of the settling material explain the sedimentary hill-vs.-Far-Field differences for the respective biochemical parameters?
The collected settling particulate material was analysed for the same organic and inorganic parameters as the sediments. POC and PN contents increased in the settling particles as a result of the arriving bloom material (Fig. S5c). If this material had a detectable impact on the
19

456
461
462
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464
457
458
459
460
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466
450
451
452
453
454
455
446
447
448
449
440
441
442
443
444
445 hill, where it would arrive before it arrives in the Far Field, then, one would expect the hill sediments to show higher OC and N contents than the Far Field sediments. What we see, however, is the opposite (Fig. 3a,b). This result indicates strongly that the OC- and N-content difference between the hill and Far-Field sites is a spatial rather than a temporal one.
An analogous line of reasoning applies to the AA / HA and Gluam / Galam ratios. Molar
AA / HA ratios at 569 mab did not show any clear temporal changes and remained near ~ 15, except for the last week of the deployment period where there was a slight drop to ~ 13
(Fig. S5d). So, if anything, the arriving fresh material is characterised by lower AA/HA ratios. If this material had a detectable impact on the hill one would expect the hill sediments to show lower AA/HA ratios than the Far-Field sediments. However, AA/HA ratios in surface sediments were higher on the hill than in the Far Field (Fig. 3e). This result indicates strongly that the
AA/HA difference between the hill and Far-Field sites is also a spatial rather than a temporal one. And, while the Gluam / Galam ratios in settling material increase with the arrival of the bloom (Fig. S5d), Gluam / Galam ratios in surface sediments tend to be lower on the hill than in the Far Field (Fig. 3f), again, indicating a spatial rather than a temporal explanation.
The other parameters of interest that displayed strong temporal variability and were clearly higher (Asp /

-Ala, Glu /

-Aba, and AA mol-% of Met) or lower (B / (B + A)) on the hill than in the Far Field did increase and decrease, respectively, in the settling material with the arrival of the fresh settling bloom material (Fig. S5e,f). So it could be argued that the values of these parameters on the hill are due to temporal (bloom-related) rather than spatial (hill-related) variability. However, the above AA-related parameters showed similar trends in surface sediments on the hill and at the Far Field SW site (Fig. 4). Far Field SW is at virtually the same depth as the other Far Field sites but is located in the sector of the hill-induced flow-field asymmetry that is characterised by increased current speeds (Fig. S2). Moreover, the Far Field
SW site is within only a few hundreds of meters of a smaller-scale topographic elevation that is associated with near-critical and supercritical slope angles (relative to the semidiurnal tide) and high tidally related maximum current speeds (Fig. S1). That is, Far Field and Far Field SW are
20

467
468
469
470
471
472 at the same depth but differ in their near-seafloor fluid dynamics. These circumstances strongly suggest that the values of Asp /

-Ala, Glu /

-Aba, B / (B + A) and AA mol-% of Met in the surface sediments at Far Field SW are due to the increased fluid-dynamic vigorousness at this locality. Far Field SW is influenced by flow / topography interactions as is the PAP hill. And this, in turn, indicates that the values of the above AA-related parameters on the hill are also due to the fluid dynamics on the hill rather than the arrival of the settling plankton bloom.
487
488
489
490
483
484
485
486
491
492
477
478
479
480
481
482
473
474
475
476
Timing of bloom arrival vs. sediment and photographic sampling ---- Further evidence can be derived from the timing of sediment and photographic sampling relative to the timing of the bloom arrival. The two aforementioned DOS deployments were carried out near the summit plateau at between ~ 600 and ~ 900 m above the seafloor of the abyssal plain. At that time, TPM and POC fluxes from the interior ocean had already increased ~ 3-fold relative to the period before the start of the arrival of the settling plankton bloom (Fig. S4). However, about one week later, TPM and POC fluxes at 569 mab were ~ 7 and ~ 9 times higher than during the period before the arrival of the bloom. Overall, the timing of events suggests that, during the photographic surveys, the DOS sites may have been influenced by moderate input of bloom material but just missed (by a few days ?) the arrival of the peak downward flux of particulate matter from the interior ocean.
Sediments in the Far Field were repeatedly sampled at different points in time, capturing the full range of input fluxes of settling particulate matter from the interior ocean (Fig. S4). Despite these different input fluxes, the reported compositional differences between sediments on the hill and sediments in the Far Field remain clear. This may be due to the fact that most Far-Field sediments were collected before the arrival of the highest settling fluxes and all hill samples were collected within a relatively short time frame of ~ 5 days at only moderately high incoming particle fluxes (Fig. S4).
Overall, it would therefore seem that the spatiotemporal sampling design ensured that most surface sediments that were sampled for this study remained unaffected by the highest
21

493
494 particulate-matter fluxes from the interior ocean that were measured towards the end of the study period.
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503
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511
512
513
514
515
516
517
518
519
520
499
500
501
502
495
496
497
498
Summary and conclusion ---- There are three main findings. (1) To explain the sediment-biogeochemical differences between the hill and the Far Field through the arrival of the sedimenting bloom particles, the OC and N contents and AA_C / OC, HA_C / OC, AA / HA and Gluam / Galam ratios in surface sediments on the hill would need to be higher than in the
Far-Field sediments. However, either there is no clear difference (as in the case of AA_C / OC) or the opposite is true (all other parameters). (2) The Far-Field sites on the one hand and the
Far-Field-SW site on the other hand are at virtually the same depths. But, in terms of the sediment-biogeochemical parameters, Far-Field SW has more in common with the hill sites than with the Far-Field sites. This is likely due to the fluid dynamics at Far Field SW being influenced by the flow-field asymmetry of the PAP hill and also by a nearby, smaller topographic feature. This is further evidence for the predominance of spatial over temporal effects that are reflected in the sediment data of this study. (3) Most sediment sampling was carried out at low and moderate input fluxes of bloom material. The small amounts of macroscopically visible phytodetrital particles in seafloor photographs from the summit area support the notion that the freshly introduced material had little if any influence on the sampled surface sediments of this study.
All these findings, taken together, support the conclusion that the hill-versus-Far-Field differences in the sedimentary organic-matter constituents (Fig. 3 and 4 of the main text) are in fact a spatial (hill-induced) rather than a temporal (bloom-related) phenomenon.
Fig. S6. Influence of the abyssal hill on measures of sedimentary organic matter quantity and composition. (a) Organic carbon (OC) and (b) nitrogen (N) contents as weight-percentages of dry sediment. (c) Amino-acid- (AA-) and (d) hexosamine- (HA-)associated carbon as a weightpercentage of OC. (e) Molar ratio of total AAs and total HAs. (f) Molar ratio of the HAs glucosamine (Gluam) and galactosamine (Galam). --------------------→
22

521
23

522 4. Figures identifying individual sediment-sampling stations and cores
523
524
525
526
For clarity, Figures 3 and 4 in the main text distinguish only three groups of sites: Far-Field sites, Far Field SW, and the hill sites. To reveal the full information, companion Figures S6 and
S7 are included here, identifying profiles from all individual sediment cores and sampling stations.
527
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529
530
531
532
533
534
535
536
Fig. S7. Influence of the abyssal hill on amino-acid-(AA-)related measures of organic-matter fractionation. The figure legend of Figure S6 also applies to this figure. (a) Molar ratios of the protein-AA aspartic acid (Asp) and the non-protein-AA

-alanine (

-Ala).

-Ala is viewed as a decompositional product of Asp. (b) The AA degradation index (DI) sensu Dauwe and
Middelburg (1998) and Dauwe et al. (1999): higher (less negative) values are thought to indicate less advanced decomposition. (c) Molar ratio B / (B + A) of basic AAs (positively charged side chains; B = histidine + arginine + lysine) and the sum (B + A) of basic and acidic
AAs (negatively charged side chains; A = Asp + Glu). (d) Mol-percentage in all AAs of the sulphur-containing AA methionine (Met).
24

537
538
539
In addition, Figure S8 shows the PIC profiles for all individual stations and sediment cores.
The average PIC data in Figure 2d of the main text were derived from the bioturbated layers that had thicknesses of up to 10 cm as can be seen in Figure S8.
540
541
542
543
544
545
546
547
548
549
550
Fig. S8. Sediment profiles of particulate inorganic carbon (PIC, calcium carbonate). Scanningelectron-microscopic images of selected samples revealed that the
PIC consists of foraminiferal and coccolithophorid material. The figure legend of Figure S6 also applies to this figure.
551
552
553
554
555
556
557
558
559
560
561
Finally, Figure S9 combines additional information on biogeochemical sediment parameters that were mentioned but not shown in the main text of this paper (total amino-acid (AA) and hexosamine (HA) concentrations; AA-bound nitrogen (AA_N/N) and HA-bound nitrogen
(HA_N/N) as weight-percentages of total sedimentary nitrogen; and the molar ratio of the protein-AA glutamic acid (Glu) and non-protein-AA and

aminobutyric acid (

Aba)).
Main results are summarised in Tab. 1 below. This table also contains measures of the robustness of hill-versus-Far-Field differences of the aforementioned sedimentary organicgeochemical parameters. Robustness was tested with the one-sided
Whitney test) (Sachs, 1992).
U test (Wilcoxon, Mann,
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564
565
566
567
568
Fig. S9. Biogeochemical sediment parameters that were mentioned but not shown in the main text. The figure legend of Figure S6 also applies to this figure. (a) Total amino-acid (AA) concentrations as a mass fraction of dry sediment; (b) hexosamine (HA) concentrations as a mass fraction of dry sediment; (c) AA-bound nitrogen (AA_N/N) as weight-percentages of total sedimentary nitrogen; (d) HA-bound nitrogen (HA_N/N) as weight-percentages of total sedimentary nitrogen; (e) molar ratio of the protein-AA glutamic acid (Glu) and non-protein-AA

aminobutyric acid (

Aba). ---------------------
→
25

569
570
26

571
572
573
574
575
576
577
578
579
580
Because of a sediment-depth-dependence of some of the parameters, the data for each parameter were grouped according to the layers 0-2, 2-5 and 5-10 cm of the sediment column.
For each layer and each parameter, the data were then grouped into hill and Far-Field data.
These two groups were then compared with the U test. The measure of robustness of the difference between hill and FarField data is the ‘probability of error’, 
. That is, for each parameter, three
 values were calculated for the three sediment layers. In the main text, these values for the layers 0-2, 2-5 and 5-10 cm are reported according to the notation

0-2
,

2-5 and

5-10
, respectively. Values higher than

= 0.1 were viewed as a representation of statistically insignificant ( ‘s.i.’ in Tab. S1) differences between hill and Far-Field data. Cases where data numbers were insufficient for the U test are indicated by ‘n.a.’ (not applicable) in Tab. S1.
581
582
27

583
584
585
Table S1. Comparison of hill and Far-Field sampling sites. The text for more details.
Parameter max. current speed (cm s -1 ) max. flow indicator I
F
210 Pb focussing factor
Grain-size fraction > 125
 m
(weight-%)
PIC (weight-%)
OC (weight-%)
0 - 2 cm
2 – 5 cm
5 – 10 cm
N (weight-%)
0 - 2 cm
2 – 5 cm
5 – 10 cm
AA_C / OC (weight-%)
0 - 2 cm
2 – 5 cm
5 – 10 cm
HA_C / OC (weight-%)
0 - 2 cm
2 – 5 cm
5 – 10 cm
AA / HA (molar ratio)
0 - 2 cm
2 – 5 cm
5 – 10 cm
Gluam / Galam (molar ratio)
0 - 2 cm
2 – 5 cm
5 – 10 cm
Asp /

-Ala (molar ratio)
0 - 2 cm
2 – 5 cm
5 – 10 cm
DI
0 - 2 cm
2 – 5 cm
5 – 10 cm
Met (mol-% of AA)
0 - 2 cm
2 – 5 cm
5 – 10 cm
B / (B + A) (molar ratio)
0 - 2 cm
2 – 5 cm
5 – 10 cm
Hill sites
9.0 ± 0.4 – 17.0 ± 3.3
0.54 ± 0.03 – 1.29 ± 0.26
0.37 ± 0.15 – 1.17 ± 0.29
22.5 ± 1.6 – 39.1 ± 0.5
54 ± 2 – 77 ± 0.04
0.21 ± 0.06
0.18 ± 0.08
0.18
0.040 ± 0.010
0.036 ± 0.011
0.036
7.8 ± 2.0
7.2 ± 3.4
5.8 ± 1.9
1.0 ± 0.2
1.4 ± 0.4
1.5 ± 0.6
11.6 ± 1.5
8.3 ± 1.6
5.5 ± 0.9
0.88 ± 0.11
0.91 ± 0.08
1.22 ± 0.74
7.6 ± 1.1
7.6 ± 1.4
6.0 ± 2.5
-1.34 ± 0.27
-1.75 ± 0.32
-1.46 ± 0.17
0.561 ± 0.173
0.586 ± 0.316
0.405 ± 0.370
0.267 ± 0.011
0.247 ± 0.025
0.257 ± 0.056
28
Far-Field
8.0 ± 0.1
0.71 ± 0.07
1.03 ± 0.33
7.1 ± 0.7
63 ± 2
0.40 ± 0.07
0.35 ± 0.05
0.29 ± 0.04
0.061 ± 0.013
0.053 ± 0.009
0.046 ± 0.008
8.3 ± 0.8
7.4 ± 0.8
6.6 ± 0.8
2.0 ± 0.3
1.9 ± 0.2
1.9 ± 0.3
6.0 ± 0.6
5.5 ± 0.7
4.9 ± 0.4
1.07 ± 0.06
1.03 ± 0.04
0.98 ± 0.06
6.1 ± 0.5
5.7 ± 0.4
4.9 ± 0.3
-1.49 ± 0.11
-1.60 ± 0.08
-1.69 ± 0.18
0.107 ± 0.061
0.152 ± 0.121
0.324 ± 0.390
0.311 ± 0.016
0.311 ± 0.019
0.304 ± 0.021
-
remark
-
-

= 0.005

= 0.025

= 0.05

= 0.01

= 0.025 s.i.

= 0.025

= 0.025

= 0.01 s.i. s.i.

= 0.05

= 0.005

= 0.025 s.i.

= 0.005

= 0.025

= 0.025
-

= 0.001

= 0.005 n.a.

= 0.005

= 0.025 n.a. s.i. s.i. s.i.

= 0.005

= 0.025

= 0.05

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