2 Surficial Sediments of the Chatham Rise: General Characteristics

Volume Two May 2014

Appendix 12

Natural Sedimentation on the Chatham Rise (Nodder 2013)

Natural Sedimentation on the Chatham Rise

Prepared for Chatham Rock Phosphate LLtd

August 2012 (Updated April 2013)

Authors/Contributors: Scott D. Nodder

For any information regarding this report please contact: Scott Nodder Group Manager/Marine Geologist Ocean Geology +64-4-386 0357 [email protected]

National Institute of Water & Atmospheric Research Ltd 301 Evans Bay Parade, Greta Point Wellington 6021 Private Bag 14901, Kilbirnie Wellington 6241 New Zealand

Phone +64-4-386 0300 Fax +64-4-386 0574

NIWA Client Report No: WLG2012-42 Report date: August 2012 (Updated April 2013) NIWA Project: CRP12302/6

Remote sensing image of spring chlorophyll a concentrations in the New Zealand region [NASA/Orbimage/NIWA] (left) and sediment trap used for measuring sinking particle fluxes. [Scott Nodder, NIWA].

© All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client.

26 April 2013 8.06 p.m.

Contents

Executive summary ...... 5

1 Objectives ...... 7

2 Surficial sediments of the Chatham Rise: general characteristics ...... 7 2.1 Grain-size distribution ...... 7 2.2 Sediment composition ...... 7 2.3 Sediment composition ...... 8 2.4 Carbonate content ...... 9 2.5 Organic content ...... 9 2.6 Sedimentation rates ...... 9 2.7 Near-surface geological formations and authigenesis ...... 10

3 Natural sedimentation: origins and processes ...... 10 3.1 Vertical fluxes ...... 11 3.2 Horizontal fluxes ...... 16 3.3 Authigenesis ...... 17

4 Water column suspended particles and turbidity ...... 18

5 Comparison to other “deep-sea” studies ...... 23

6 Conclusions ...... 24

7 Acknowledgements ...... 25

8 References ...... 25

Figures Figure 2-1: NIWA 1:1 million scale Sediment chart, highlighting surficial sediment distributions on the Chatham Rise and in the Bounty Trough (McDougall, 1982). 8 Figure 3-1: Particulate fluxes in subtropical waters, just to the north of the Chatham Rise, east of New Zealand (station U940), in austral autumn, 1992 (reproduced from Figure 4, Nodder, 1997). 12 Figure 3-2: Average total mass (left) and particulate phosphorus (right) fluxes in different water types across Chatham Rise in winter (A) and spring (B). 13 Figure 3-3: Re-plotted data from Zhou et al. (2012) showing the spatial distribution of 234Th and POC fluxes on Chatham Rise. 15 Figure 4-1: Full water column nephelometer profiles on the crest (U941, left) and northern flank of the Chatham Rise (U943, U944 and U949). 19

Natural Sedimentation on the Chatham Rise

Figure 4-2: Water column profiles of suspended particulate matter concenttrations on the northern flank of the Chatham Rise. 20 Figure 4-3: Fluorescence (top left panel) and beam transmission (right) profiles from CTD data collected on the crest of the Chatham Rise. 22

Reviewed by Approved for release by

Helen Neil Julie Hall

Natural Sedimentation on the Chatham Rise

Executive summary The report summarises the natural sedimentation processes on the Chatham Rise in order to provide background information to the proposed phosphorite sea-bed mining operations of Chatham Rock Phosphate (CRP) Ltd. The sediments on the crest of the Chatham Rise are predominantly phosphorite-bearing, glauconitic, fine- to medium-grained, foraminiferal sandy muds or muddy sands, with subsidiary phosphorite nodules (typically part of the >5% gravel component), rock fragments, volcanic ash, clay minerals and biogenic material. Calcium carbonate percentages of these surficial sediments are typically 30-40% with organic carbon levels of 0.5-1%. These surface sediments are considered to represent a condensed sequence, representing a long period of time preserved in a relatively thin layer of material (typically <10 m thick).

Downward vertical fluxes of material on the crest of the Chatham Rise within the Subtropical Front are generally elevated compared to the water masses on either side of the rise, reflecting the highly productive nature of the frontal zone. Vertical fluxes on the rise are in the order of 180-500 mg/m2/d, but have been measured as high as 1800 mg/m2/d close to the sea-floor. These vertical flux values indicate that the Chatham Rise has fluxes that are similar in magnitude to other temperate, continental margin and open ocean environments.

Sinking particles are affected by currents that can move them horizontally away from their point of origin, and such near-bed currents can also be sufficient to resuspend and transport bottom sediments. Measurements of water column turbidity, including recent mooring data collected by IX Survey for CRP Ltd, show that increases in turbidity are a prominent feature close to the Chatham Rise sea-floor, with values as high as 0.55 mg/l on the crest of the rise.

While the report summarises the information available on natural sedimentation on the Chatham Rise crest, there is actually very limited temporal and spatial information on the magnitude and timing of episodic flux events in this region. In particular, information is lacking on how water column particulate populations and sea-floor sediments on the crest interact with the dynamic physical environment within the Subtropical Front over a variety of temporal (days to years) and spatial (1 to 100’s of km) scales.

Natural Sedimentation on the Chatham Rise 5

1 Objectives In July 2012, the National Institute of Water and Atmospheric Research (NIWA) Ltd was contracted by Chatham Rock Phosphate (CRP) Ltd to undertake a review of natural sedimentation on the Chatham Rise. The review is designed to provide background information on the present-day sedimentary system on the rise in the context of CRP’s proposed sea-bed mining operations for phosphorite nodules.

The objectives of this report are to:

. Describe natural sedimentation of particulate matter on the Chatham Rise;

. Overview natural sediment producing processes and nature of particulates;

. Overview studies/identify locations where turbidity/TSS etc. data from on the Chatham Rise and nearby;

. Overview water column particulate data/turbidity from the various data sources above, including IX data;

. Overview sedimentation studies and the data available from those studies; and

. Comment on ‘deep sea’ data (similar depths) where data might provide useful comparative data. 2 Surficial sediments of the Chatham Rise: general characteristics

2.1 Grain-size distribution The surface sediments on the Chatham Rise crest are predominantly sandy muds or muddy sands (~55-60% >63 µm), typically comprising 25-30% silt (63-4 µm, %mud = %silt + %clay) and ~5-10% clay (<4 µm) in the mud fraction (<63 µm) (e.g., Table 1 in Grove et al., 2006; also Appendix in Nodder et al., 2011) (also see comparable data in McDougall (1982) and von Rad and Rösch (1984)) (Figure 2-1). Detailed mapping by Lawless (2012) showed that, based on Folk’s (1968) classification, on the rise crest between 177°E and 180° the surficial sediments are predominantly sands on shallow highs (<250 m water depth), such as Reserve Bank, grading to silty sands and sandy silts at deeper sites (>250 m), with >5% gravel (mainly phosphorite nodules) east of 179°E. From previous studies, phosphorite nodules are typically in the 0.5->1 mm size-fraction, up to maximum nodule diameters of 50- 200 mm, increasing in size down-core towards the underlying Oligocene chalks (see below) (von Rad and Rösch, 1984).

2.2 Sediment composition The surficial sediments of the Chatham Rise crest dominantly comprise phosphorite- bearing, glauconitic, fine- to medium-grained sandy muds and muddy sands (e.g., Norris, 1964; Pasho, 1976; Kudrass and Cullen, 1982; von Rad and Rösch, 1984; McDougall, 1982; Grove et al., 2006; Nodder et al., 2007, 2011, 2012; Lawless, 2012). Subsidiary carbonate fractions include, for example, planktic and benthic foraminifera, molluscs and echinoderm fragments. Minor, though significant, contributions are made by phosphorite nodules (Cullen,

Natural Sedimentation on the Chatham Rise 7

1980, 1987; sometimes as much as 70% in the >1 mm fraction, von Rad and Rösch 1984) and detrital rock fragments that are sourced locally from exposed basement rock (Cullen, 1965; Wood et al., 1989) or ice-rafted debris transported by and deposited from ancient icebergs during previous glacial periods (Cullen, 1962). Other components include faecal pellets and biogenic constituents (diatoms, coccoliths, radiolarians), volcanic ash (Norris, 1964; Barnes and Shane 1992), glauconite pellets (von Rad and Rösch, 1984; Lawless, 2012) and clay minerals (Norris, 1964; Lawless, 2012).

Figure 2-1: NIWA 1:1 million scale Sediment chart, highlighting surficial sediment distributions on the Chatham Rise and in the Bounty Trough (McDougall, 1982).

2.3 Sediment composition The surficial sediments of the Chatham Rise crest dominantly comprise phosphorite- bearing, glauconitic, fine- to medium-grained sandy muds and muddy sands (e.g., Norris, 1964; Pasho, 1976; Kudrass and Cullen, 1982; von Rad and Rösch, 1984; McDougall, 1982; Grove et al., 2006; Nodder et al., 2007, 2011, 2012; Lawless, 2012). Subsidiary carbonate fractions include, for example, planktic and benthic foraminifera, molluscs and echinoderm fragments. Minor, though significant, contributions are made by phosphorite nodules (Cullen, 1980, 1987; sometimes as much as 70% in the >1 mm fraction, von Rad and Rösch 1984) and detrital rock fragments that are sourced locally from exposed basement rock (Cullen,

8 Natural Sedimentation on the Chatham Rise

1965; Wood et al., 1989) or ice-rafted debris transported by and deposited from ancient icebergs during previous glacial periods (Cullen, 1962). Other components include faecal pellets and biogenic constituents (diatoms, coccoliths, radiolarians), volcanic ash (Norris, 1964; Barnes and Shane 1992), glauconite pellets (von Rad and Rösch, 1984; Lawless, 2012) and clay minerals (Norris, 1964; Lawless, 2012).

2.4 Carbonate content

The calcium carbonate concentration (%CaCO3) in surficial sediments on the crest of the Chatham Rise is typically 30-40% (McDougall, 1982; Grove et al., 2006; Nodder et al., 2007, 2011; Lawless, 2012), with lower concentrations (10-20%) at relatively shallow sites, such as Reserve Bank (Lawless, 2012) and increasing to 40-50% at water depths deeper than ~500 m (Grove et al., 2006; Nodder et al., 2007) and higher (>80% carbonate) on the very shallow highs, such as Mernoo Bank, Veryan Bank and the Chatham Island platform (McDougall, 1982). Based on only two samples from the Chatham Rise crest, Hayward et al. (2002) indicated that there were estimated to be 10000 to 30000 calcareous benthic foraminifera per gram of CaCO3, with the rise crest also recognised as two specific benthic foraminiferal assemblage/environmental classes. For example, western and central regions of the rise crest (bathyal assemblage B1, 450-1320 m water depths) are characterised by benthic foraminfera that respond to episodic food input, strong lateral advection and low bottom water oxygen concentrations, compared to shallower eastern fauna (shallow assemblage S1, 280-620 m, east of ~180°) that generally occur in well-oxygenated waters influenced also by near-bottom currents and the lateral input of organic matter (Hayward et al., 2002).

2.5 Organic content The organic content of the surficial sediments on the Chatham Rise crest ranges from 2-5% total organic matter (loss-on-ignition method, Grove et al., 2006; Nodder et al., 2011), equivalent to 0.5-1% in terms of organic carbon (CHN analyser method, Grove et al., 2006; Nodder et al., 2007). The labile, or “fresh”, component of organic material in these surficial sediments, as estimated by algal pigment chlorophyll a (chl a) concentrations, varies from 0.06 to 0.10 ng chl a/mg dry sediment (Grove et al., 2006; Nodder et al., 2007; Berkenbusch et al., 2011). In a general sense, organic content of the Chatham Rise sediments are enhanced on the crest of the rise, compared to the upper northern and southern flanks, which reflects the seasonal persistence of algal production and subsequent deposition within the waters associated with the Subtropical Front over the rise crest (e.g., Murphy et al., 2001) (see below). Seasonal deposition of algal material from near-surface phytoplankton blooms, especially on the southern flank, however, can distort this generality, resulting in organic-rich material being deposited to the sea-floor, especially on the upper slopes (e.g., Nodder and Northcote, 2001; Nodder et al., 2007).

2.6 Sedimentation rates Due to the condensed nature of the surficial sediments and the shallow burial and occasional exposure of Paleogene and Miocene limestone chalks on the crest of the Chatham Rise (e.g., Cullen, 1980, 1987 Lawless, 2012), it is difficult to ascertain a realistic modern-day sedimentation rate for these deposits. On the flanks of the rise, where thicker Quaternary (<1 million years old) units have been sampled, long-term sedimentation rates of 1-6 cm/1000 years have been determined for the last 12 000 years (Marine Isotope Stage (MIS) 1/Holocene) and for the last glacial age (12-27 000 years, Marine Isotope Stage 2) (Carter et

Natural Sedimentation on the Chatham Rise 9

al., 2000). It is expected that modern sedimentation rates on the crest of the Chatham Rise will be substantially less than these estimates. Similarly, Campbell Plateau, at a comparable depth range, but significantly more isolated than Chatham Rise from both terrigenous sediment supply and strong bottom current flows, has reported long-term sedimentation rates of 2-3 cm/1000 years and mass accumulation rates of 1.5-3.5 g/cm2/1000 years for carbonate and 0.1 -1 g/cm2/1000 years for non-carbonate accumulation (Neil et al., 2004). However, these Challenger Plateau sediments are totally different to those found on the Chatham Rise, being pelagic carbonate sediments, composed of coccolith-rich foraminiferal ooze (MIS 5), overlain by olive gray foraminiferal mud (MIS 2) and grading into light gray foraminiferal ooze at the sea-floor (MIS 1).

Measurements of total organic carbon (TOC) accumulation rates in sediment cores, from the flanks of the Chatham Rise, not the crest, were undertaken by Dr Liz Sikes, Rutgers University, USA. From Sikes’ unpublished data, TOC accumulation rates over the top 11 cm of sediment cores ranged from 4–7 mgC/cm2/y at ~1000 m water depth on the northern flank of the Chatham Rise, compared with ~16 mgC/cm2/y at 1300 m on the southern flank (see Nodder et al., 2003).

2.7 Near-surface geological formations and authigenesis Underlying the surficial sediments (typically <1 m-thick) are Upper Eocene-Lower Oligocene chalky limestones that are softened, presumably by intense bioturbation, which has resulted in a highly burrowed upper surface (Cullen, 1987). Conspicuous phosphorite nodules are found scattered within these surficial units and on the seafloor (Cullen and Singleton, 1977; Cullen, 1980), having developed from the erosion and phosphatisation of Lower and Middle Miocene limestones about 15-5 million years ago (Burns, 1984; Cullen, 1980, 1987; McArthur et al., 1990). Glauconite minerals are often authigenic in origin (McDougall 1982), although a recent Chatham Rise study suggests they may be best regarded as allogenic (i.e., derived from submarine erosion of Miocene limestones; Lawless, 2012), with glauconisation ages in the order of 5-7 million years (Cullen, 1967; Kreuzer, 1984; Lawless, 2012).

Older rocks are also exposed on submarine highs, such as Mernoo Bank (Permian-Triassic greywacke and possibly Cretaceous sandstones), Veryan Bank (greywacke) and Matheson Bank (schist), with localised Paleogene volcanic rocks primarily in the vicinity of the Chatham Islands and on the northern flank east of 180° (see Wood et al., 1989). 3 Natural sedimentation: origins and processes The variable composition and nature of sediments on the Chatham Rise indicate that natural sedimentation in this region has a number of origins and is affected by a range of processes.

Sediment origins include:

. remobilisation and redistribution of relict sediments;

. in situ production by biomineralisation and/or authigenesis;

. production of organic material by surface water photosynthesis (phytodetritus) and foodweb modifications (e.g., faecal pellets, marine aggregates, zooplankton carcasses);

10 Natural Sedimentation on the Chatham Rise

. inorganic material (e.g. foraminifera, coccoliths, radiolarians);

. clay mineral deposition (terrigenous and airborne, including volcanic ash); and

. erosion of local rock sources (e.g., Norris, 1964; Pasho, 1976; von Rad and Rösch, 1984; Nodder et al., 2007; Lawless, 2012).

Sedimentary processes include:

. active settling of material through the water column (vertical flux);

. resuspension and transport by near-bed currents (horizontal flux); and

. authigenesis (chemical precipitation) (e.g., McDougall (1982), although recent research advocates a near-absence of modern-day authigenesis at least for the pelletised glauconite fraction (Lawless (2012)).

3.1 Vertical fluxes In the ocean, particles are formed and/or deposited in the surface ocean, are modified by physico-chemical and/or biological processes and then eventually sink into the ocean’s interior. During their passage to the sea-floor, sinking particles are further modified by grazing organisms (zooplankton), microbial decomposition and chemical remineralisation (dissolution, chemical transformation). While this sinking process can be regarded as essentially a vertical process under the influence of gravity, currents in the ocean serve to translate the sinking particles away from their original point of origin. The degree to which this translation occurs is largely a function of the particle’s sinking speed, such that fast sinking particles (typically >100 m/day) will be deposited closer to source than slower sinking particles. In addition, the longer a particle remains suspended in the water column the greater propensity for modification, repackaging and remineralisation.

Marine particles comprise both organic and inorganic constituents, which often aggregate together to form “marine snow”. Organic material includes primary algal material (e.g., phytoplankton cells, chains and aggregations that may be alive, senescent or dead) and derived products, such as carcasses and faecal pellets from zooplankton and nekton. Inorganic material includes hard-shelled biological components, such as silica shell- producing diatoms, silico-flagellates (both are types of phytoplankton) and radiolarians (zooplankton), carbonate shell-producing coccolithophores (phytoplankton), foraminifera and pteropods (both zooplankton), and lithogenic clays and mineral grains. In many cases, the ballasting of low density organic particle aggregates with inorganic material is crucial in increasing the density and hence the sinking speed of marine snow.

There have been only limited measurements of vertical sinking flux and suspended particle concentrations in the Chatham Rise area. To gain a quantitative measurement of vertical fluxes, the standard method is to deploy sediment traps at various depths in the water column on either free-drifting/surface-tethered or bottom-anchored moored arrays. In 234 addition, geochemical proxies for vertical flux can be used, such as Th and Baexcess inventories.

The first deep ocean sediment trap study conducted in the New Zealand region was by Nodder (1997) who showed that on the northern slopes of the Chatham Rise total particle

Natural Sedimentation on the Chatham Rise 11

mass fluxes increased from ~60 to 160 mg/m2/d over water depths between 200 and 500 m (CR2053, station U940, start 42° 40.49’ S, 178° 59.28’ E, finish 42° 22.84’ S, 179° 22.90’ E) (Figure 3-1). These measurements were made using a free-floating sediment trap array, deployed for 3 days in April 1992.

Figure 3-1: Particulate fluxes in subtropical waters, just to the north of the Chatham Rise, east of New Zealand (station U940), in austral autumn, 1992 (reproduced from Figure 4, Nodder, 1997).

12 Natural Sedimentation on the Chatham Rise

Nodder and Alexander (1998) undertook duplicate free-floating sediment trap deployments in the Subtropical Front on the Chatham Rise as part of a wider study investiigating the fluxes of carbon within marine ecosystems in the main water masses of the New Zealand region (CR3009, CR3014; e.g., Bradford-Grieve et al., 1999). This study found thhat on the crest of the Chatham Rise total particle mass fluxes measured at 210 m water depth were substantially higher than those at 110 m aat one station (U467, 43° 29.29’ S, 176° 26.80’ E) in winter 1993 (~average 190 and 500 mg/m2/d at 110 and 210 m, respectively, about 150 m above the seafloor) (Figure 3-2). In comparison, in spring 1993 fluxes were only slightly elevated at 220 m water depth, compared to 110 m, at one free-floating trap station (X477, ~190-300 mg/m2/d), compared to a significant increase observed at the othher duplicate station (X478), where near-surface fluxes measured at 100 m were ~180 mg/m2/d, increasing to just over 1800 mg/m2/d at 220 m (Nodder and Alexander, 1998).

Figure 3-2: Average total mass (left) and particulate phosphorus (right) fluxxes in different water types across Chatham Rise in winter (A) and spring (B). Error bars are standard errors (S.E. = standard deviation/n, where n = number of samples used in calculattion of mean values).. SA = Subantarctic (~46°S 174°E), STC = Subtropical Convergence/Chatham Rise crest (~43.5°S 176.5°E), ST = Subtropical (~41.5°°S 179°E) (reproduced from Figures 4 and 5, Nodder and Alexander, 1998).

Other noteworthy particle flux studies incclude Nodder and Gall (1998) and Nodder et al. (2007) who investigated pigment fluxes in the Subtropical Front, and the moored trap studies on the southern and northern flanks of the Chatham Rise reported by Noddder and Northcote (2001). Nodder and Gall (1998) showed that pigment fluxes were dominated by chlorophyll a (a proxy for algal biomass), fuxoxanthin (proxy for diatom biomass) and phaeophorbides (degraded pigments, such as those found in faecal pellets), highlighting the dual importance

Natural Sedimentation on the Chatham Rise 13

of primary production-derived organic material and secondary and/or resuspended pigmented products as potential food sources for the benthos on the Chatham Rise.

Nodder and Northcote (2001) described a deep-ocean sediment trap mooring programme that deployed traps at 300 and 1000 m depths for a year (1996-1997) at 42° 42’ S on the northern flank of the Chatham Rise and 44° 37’ S on the southern flank on longitude 178° 40’ E. The lowermost trap at both locations was deployed about 500 m above the seafloor, and in both deployments increases in mass flux between 300 and 1000 m were observed, and related to sediment resuspension and differential particle settling. Mass fluxes at 300 m on the southern flank in spring ranged from 17 to maximum values of 290 mg/m2/d, compared to <2 to 1060 mg/m2/d on the northern flank. High spring fluxes were also observed on the southern flank at 1000 m (~40-110 mg/m2/d), which were between 20-200% of the fluxes measured at 300 m. On the northern flank at 1000 m, very high fluxes persisted for almost the complete sampling record, ranging from 140-1020 mg/m2/d and were highest in spring. These high mass fluxes were associated with elevated particulate organic carbon (POC) and biogenic silica fluxes at both 300 and 1000 m. Nodder and Northcote (2001) also estimated an organic particle sinking speed of ~100 m/d. Particle source areas were in the order of ~40 km (average, range 10-120 km) at the northern mooring site and 80 km to the south (range 40-120 km), although it was proposed that on the southern flanks the persistence of eastward currents over the year-long deployment might cause the particle source area to be more elongated in shape (Nodder and Northcote, 2001).

One issue with sediment trap and mooring studies is that they are restricted spatially. This could be an especially significant factor in a region such as the Chatham Rise where currents in the Subtropical Front are variable in space and time (e.g., Chiswell, 1994; Sutton, 2001; see summary below under Horizontal fluxes). A recent study by Zhou et al. (2012) attempted to determine the spatial variability of particle fluxes within the Subtropical Front using the distribution of the particle-reactive radio-isotope 234Th. These data suggest that particle export in late autumn-early winter (May-June 2008) decreased from west to east and from north to south across the Chatham Rise (Figure 3-3). In this study, POC fluxes reached highs of 15-30 mmol C/m2/d (equivalent to 180-360 mg C/m2/d) on NW Chatham Rise, decreasing to values of 10-15 in central rise areas to lows of 6-7 (70-85 mg C/m2/d) on SE Chatham Rise. The north-to-south decrease in fluxes across the rise is consistent with previous observations using moored sediment traps (Nodder and Northcote, 2001).

14 Natural Sedimentation on the Chatham Rise

Figure 3-3: Re-plotted data from Zhou et al. (2012) showing the spatial distrribution of 234Th and POC fluxes on Chatham Rise. TAN0806 sttations on the rise are shown in the upper panel, with the spatial distribution of the particle-reaactive radio-isotope 234Th fluxes att 100 m water depth (left lower panel - a), at the depth of the euphotic zone (left lower panel - b), C/Th at 100 m depth (right lower panel - c) and particulate organic carbon (POC) flux at 100 m depth (right lower panel – d). Data and plots courtesy of K. Zhou and M. Dai (State Key Lab of Marine Environmental Science, Xiamen University, Xiamen, China).

Natural Sedimentation on the Chatham Rise 15

3.2 Horizontal fluxes Generally, it is anticipated that particle fluxes will decrease with increasing depth due to particle decomposition and remineralisation. The increases in particle flux with depth observed in the sediment trap studies on the Chatham Rise can be attributed to several causes (e.g., horizontal advection, vertical migration and mid-water activities by zooplankton, patchiness of near-surface biological processes, variable sinking speeds of particles, in situ microbial processes, etc). The most likely causes of the observation of increasing particle fluxes with increasing water depth within the Subtropical Front on the Chatham Rise, however, are lateral advection and/or resuspension and remobilisation of bottom sediments, as suggested by other measurements often made at the same time as the sediment traps (e.g., Nodder, 1997; Nodder and Alexander, 1998; Nodder and Northcote, 2001; Nodder et al., 2007).

In particular, Nodder et al. (2007) showed that resuspension of bottom sediments on the crest and upper flanks of the Chatham Rise does occur, perhaps periodically on spring tidal cycles, which are dominated by the semi-diurnal M2 tide, with a period of 12.4 h (e.g., Heath, 1985; Stanton et al., 2001). Newly deposited phytodetritus on the sea-floor was found to be resuspended as near-bed current speeds exceeded 10 cm/s (equivalent to a critical shear velocity, u*crit, for the phytodetritus of 0.5 cm/s), with residual currents on the crest directed towards the south and southwest (Nodder et al., 2007). By way of comparison with inorganic particles, relationships between sediment grain-size and current speed suggest that silt-sized particles (0.01 to 0.063 mm) may be resuspended as flow speeds exceed 10-15 cm/s (e.g., McCave and Hall, 2006). Photographic evidence from camera surveys on central Chatham Rise suggests that modern-day current winnowing of surficial sediments is minimal as attested by the pervasive evidence of surface bioturbation by animals and the lack of sea- floor sedimentary features, such as ripple-marks and sand-waves (Cullen and Singleton, 1977). From this study, it also apparent that the fine-grained surficial sediments (“superficial ooze” of Cullen and Singleton, 1977) are relatively unconsolidated and can be resuspended by only minor disturbances of the sea-floor, Furthermore, the exposure of basement rocks and younger geological formations at the sea-floor suggests that, in places on the rise, there have been or are near-bed currents in operation that have removed overlying sediments.

In order to ascertain the likelihood of currents being able to resuspend and transport sea-bed sediments on the crest of the Chatham Rise, it is useful to first briefly summarise the physical attributes of the Subtropical Frontal zone in this region. Chiswell (2010) provided a summary of the physical oceanographic measurements from the Chatham Rise region, including data from fixed current meter moorings, profiling instruments (e.g., Conductivity-Temperature- Depth (CTD) sensors deployed from ships), underway ship-board instrumentation (e.g., Acoustic Doppler Current Profilers (ADCP)) and remote sensing (e.g., sea-surface height, temperature and ocean colour).

These data indicate that currents on the Chatham Rise are variable in both space and time (e.g., Heath, 1983, 1984; Chiswell, 1994, 2001; Sutton, 2001). Flows on the Chatham Rise are dominated by the lunar semi-diurnal tidal constituent M2, which strongly influences across- rise (or meridional) currents. The strongest flows on the rise crest are in an along-rise (zonal) direction and are dominated by diurnal tides (O1 and K1; Chiswell, 1994). Flows on the top of the rise are generally <20 cm s-1 (Heath, 1983, 1984; Chiswell, 1994). Heath (1984) indicates that over a 34-day sampling period at 43° 34.3’S 179° 26.9’E, maximum current flows of 43, 33 and

16 Natural Sedimentation on the Chatham Rise

24 cm/s were measured at 43, 193 and 393 m water depths, respectively (the latter 17 m above the sea-floor). A maximum along-rise flow of 37 cm/s was reported by Chiswell (1994) at his central mooring (43° 20’S 179° 00’E). Near-bed flows (~2 m above the sea-bed) of 5-10 cm/s were observed over a 1 day period at 350 m water depth on the crest of the rise (43° 26’S 178° 30’E) and were responsible for re-suspending low density organic-rich material from the sea- floor (Nodder et al., 2007).

New ADCP and current meter data collected by IX Survey for CPR at 43° 29’S 179° 20’E indicated that from measurements made by a RDI ADCP instrument deployed 10 m above the sea-floor “the current velocity is homogeneous along (sic) the water column; the current increases slightly from bottom to surface” (p. 4; IX Survey, 2012) from ~10 cm/s to 30-40 cm/s, predominantly directed north or south. Aquatec Aquadopp current meter data at 10 m above the sea-bed indicated that flows at the start of the time-series were predominantly 10-15 cm/s (IX Survey, 2012). Over the ~7 month deployment period (May-Dec 2011), there appeared to be an overall increase in current speed up to 15-25 cm/s, and sporadically as high as 40 cm/s, through most of the record from July 2011 onwards.

Flows in the Subtropical Front on the Chatham Rise crest are highly variable, displaying considerable spatial heterogeneity, with zones of flow convergence and divergence changing over at least weekly time-scales (e.g., Chiswell, 1994; Sutton, 2001; Nodder et al., 2007). ADCP data collected in October 2001 suggested a general southward flow across the rise, consistent with contemporaneous near-bed current meter measurements (Nodder et al., 2007) and previous assertions from simple diffusion-advection balance models (Heath, 1976, 1981), but this feature was not apparent in previous studies (e.g., Chiswell, 1994). Recent ocean hydrodynamic and sediment plume modelling results, completed by NIWA for CRP (Widespread Energy), are broadly comparable to these data-sets (Hadfield, 2011) (see also Hadfield et al. (2007)).

3.3 Authigenesis While not strictly a “sedimentation” process, authigenesis, which is the process by which minerals form in a sedimentary rock after its deposition, is an important depositional process in the Chatham Rise sediments. McDougall’s (1982) chart shows authigenic sediments to be a dominant component of the Chatham Rise seabed substrates, at least out to a longitude of about 179° 30’E, with sporadic patches east of here (Figure 6.1). These authigenic minerals are predominantly in the fine- to medium-grained sands (glauconite; e.g., Lawless, 2012) and coarser gravels (phosphorite; e.g., Cullen, 1987) (McDougall, 1982) (also see previous text).

While environmental conditions for authigenesis on the Chatham Rise appear to persist in the modern-day (e.g., strong upwelling of P-rich nutrients, high biologically productive surface waters, elevated organic flux to the sea-floor, sediment reducing conditions, microbial activity (see for example, Cullen, 1987; Brookfield et al., 2009), the dating of phosphorite formation (Middle Miocene, 11-5 million years ago; Burns, 1984; Cullen, 1980, 1987; McArthur et al., 1990; Hughes-Allan, 2011) and glauconite rim precipitation and pelletisation (5-7 million years ago; Cullen, 1967; Kreuze, 1984; Lawless, 2012) suggest that active authigenesis is probably not occurring presently. Hughes-Allan (2011) suggested that, based on observations in Cullen (1987), the Chatham Rise phosphorite deposit should be regarded as diagenetic in origin, rather than authigenic, due to the observation that the phosphorite was a replacement product for the carbonate host rock (?Oligocene-Miocene limestones). It is clear, however, that the processes

Natural Sedimentation on the Chatham Rise 17

of authigenesis and diagenesis are intimately linked, with phosphogenesis (i.e., “the authigenic formation of phosphate minerals”, Filippelli, 2011, p.762) involving the “diagenesis of P-bearing phases in marine sediments, the release of P to interstitial waters, the local supersaturation of P and the authigenic formation of CFA [carbonate fluoroapatite]” (Filippelli, 2011, p.762). 4 Water column suspended particles and turbidity There are little water column data published on the distribution of water column total suspended solids (TSS) in the Chatham Rise region. McCave and Carter (1997) showed a series of nephelometer profiles, extending northwards in a transect from near the base of the northern flank of the Chatham Rise onto the Hikurangi Plateau, that were characterised by a near-surface particle maxima and a well-developed bottom nepheloid layer (BNL) at water depths deeper than ~2500 m. TSS concentrations in the most developed BNL were typically less than 0.065 mg/l, with background minimum water column concentrations in the order of 0.012 mg/l.

Nephelometer profiles and suspended particulate matter measurements collected at a station on the crest of the rise (NIWA station U942, 43° 20.32’S 179° 00.03’E) indicated the presence of near-bed particle maxima up to about 150 m above the sea-floor, with measured concentrations of ~0.55 mg/l (Nodder, 1997) (Figure 4-1, Figure 4-2). Mid-water particle concentrations at 300 m depth on the northern slope also varied 5-fold over a 12-hour sampling period from 0.05 to 0.25 mg/l (stations U946-U949, 42° 42.00’S 179° 00.00’E).

These measurements demonstrate the highly variable nature of suspended particle concentrations in the vicinity of the Chatham Rise crest, as well as highlighting the relatively elevated amounts of material in the water column on the rise, compared to surrounding waters (e.g., McCave and Carter, 1997), and the occurrence of BNLs on the crest that host particle concentrations that are 1.5 to 2 times elevated compared to surface concentrations (Figure 4-2).

18 Natural Sedimentation on the Chatham Rise

Figure 4-1: Full water column nephelometer profiles on the crest (U941, leftt)) and northern flank of the Chatham Rise (U943, U944 and U9449). Spikes on the profiles below ~500 m are thought to be electronic artefacts (reproduced from Figure 3, Nodder, 1997). Note the elevation of particle concentrations near the sea-bed, especially on profile U491, corressponding to the high particle concentrations actually measureed on water samples collected on a subsequent CTD cast, U492 (Figure 4.2).

Natural Sedimentation on the Chatham Rise 19

Figure 4-2: Water column profiles of suspended particulate matter concentrations on the northern flank of the Chatham Rise. The shallowest data is from station U942 on the crest. Stations U946-U949 were conducted at the same position over a 12 hour period (reproduced from Figure 2, Nodder, 1997).

20 Natural Sedimentation on the Chatham Rise

Recent turbidity data were collected from a mooring deployed for a 7 month period in 2011 (May-December) on the Chatham Rise crest at 43° 29.003’S 179° 20.099’E by IX Survey for CRP (IX Survey, 2012). The full water column ADCP backscatter data were used to derive a turbidity estimate in Nephelometer Turbidity Units (NTU) based on the linear relationship:

Sv (in dB) = 14 x (Turbidity (in NTU)) + 61 (1) with Sv derived from the ADCP raw data using equations of Dienes (1999). Unfortunately, in the IX Survey (2012) report, the regression co-efficient for the linear relationship is not provided in order to ascertain the relative accuracy of the derived formula. In any case, these data suggest that water column acoustic turbidity varies diurnally and vertically through the water column (IX Survey, 2012). Diurnal variations can probably be largely explained by vertically migrating zooplankton (e.g., McClatchie et al. 2004), while apparently elevated turbidity levels, especially in the surface waters near the end of the time-series are probably related to increasing primary productivity, associated with the spring bloom, at this time (e.g., Murphy et al., 2001). For most of the turbidity record derived from the ADCP data, however, there is a heightened intensity of backscatter response over most of the water column from about 50 m above the sea-floor to the surface that persists from June to mid-September 2011. It is presently uncertain what might be the cause of this prominent feature.

In addition, turbidity data from two Aquatec Aqualoggers deployed at water depths of 359 and 373 m at 43° 29.043’S 179° 20.076’E on a near-by mooring to the ADCP show differences in relative turbidity, presented as Formazin Turbidity Units (FTU). The time-series records are generally non-coherent, with spikes in turbidity at one sensor not matched by similar peaks at the other, although both records show low levels of turbidity over the first ~3.5 months. The near-bottom sensor (~10 m above the sea-floor) shows a progressive increase in turbidity at the end of the time-series from November through to the end of the time-series in early December 2011. Over the same time period, the upper sensor appears to show an increase in the number of turbidity spikes, but no overall increase from background levels.

From the data provided in the IX Survey (2012) report, it is difficult to relate these turbidity observations to the moored current meter data (ADCP and Aquadopp). With regard to the Aquadopp data, there does not appear to be a substantial increase in current speed (i.e., mean±standard deviation for the period before 8/11/11 is 17.4±7.6 cm/s cf. 17.5±7.6 cm/s over the period of elevated turbidity), which could account for the observed increase in turbidity over this time period (i.e., 1.01±0.69 prior to 8/11/11 cf. 4.81±2.65 FTU after this date). The high turbidity readings from the lower Aqualogger (>20 FTU) all occur at current velocities <22 cm/s, with the highest turbidity concentrations (>25 FTU) occurring when currents were <13 cm/s, so there is no obvious intuitive relationship between suspended particle concentrations and current speed from this time-series. It is expected, however, that the elevated turbidity observed at the lower sensor would be due to the resuspension and transport of bottom sediments by strong bottom currents (i.e., probably at least >10 cm/s for fine, organic-rich material (e.g., Nodder et al., 2007), and as much as 20-30 cm/s for sand- sized particles (e.g., Heath, 1984), especially as the increased turbidity at the lower sensor was not matched by contemporaneous heightened activity at the upper sensor.

Finally, while the turbidity data provided by the IX Survey moorings are useful in a relative sense, since there are no calibration files, it is impossible to relate the measurements to

Natural Sedimentation on the Chatham Rise 21

actual TSS concentrations (e.g., Nodder, 1997). Furthermore, direct comparisons between the two different instruments are not possible because the turbidity units are in derived NTU for the ADCP and in FTU for the Aqualoggers. The absence of such calibration information (e.g., lab or field-based concentration-NTU curves) suggests that while the turbidity data, as reported by IX Survey, provides a relative estimate of suspended particulate material (and biota) over the time period of the mooring deployments, it has only limited value in terms of characterising the absolute water column TSS concentrations in the vicinity of the proposed CRP prospecting and mining operations. In addition, none of the studies to date have attempted to discern the particle-size spectra of suspended particulate matter populations, which has implications on the expected sinking rates and hence residence times of particles in the ocean (e.g., Eittriem, 1984; Burd and Jackson, 2009).

A compilation of beam transmissivity data collected by NIWA during a variety of voyages over the last 12 years (1999-2011) shows that near-bed particle concentration maxima are apparent on most CTD casts conducted on the Chatham Rise crest in the vicinity of CRP’s area of interest (Figure 4-3). Unfortunately, most of these data are not calibrated by in situ measurements of suspended particulate matter concentration, but they do show that, in a relative sense, there are persistent elevated particle concentrations within as much as 80- 100 m above the seabed, although well-developed BNLs typically extend only 40-60 m in height off the sea-floor. Interesting, there are occasional times when these increases in particle concentrations are matched by near-bed increases in fluorescence, suggesting that photosynthetic material (phytodetritus) is probably involved in the resuspension of sea-floor particles (e.g., Nodder et al., 2007).

Figure 4-3: Fluorescence (top left panel) and beam transmission (right) profiles from CTD data collected on the crest of the Chatham Rise. Data are from 17 CTD casts undertaken between 1999-2011 (coloured dots on bottom left panel; colours are matched on the fluorescence and beam transmission profiles) (data from M. Walkington, NIWA CTD Archive).

22 Natural Sedimentation on the Chatham Rise

5 Comparison to other “deep-sea” studies The Chatham Rise crest lies at upper bathyal depths (200-600 m) and beneath a highly productive frontal zone that extends from one side of the Chatham Rise to the other (approximately 100 km; Heath, 1985; Uddstrom and Oien, 1999; Sutton, 2001). It, therefore, has few direct analogues on a global scale. The current regime within the frontal zone is complex and highly variable in time and space (e.g., Chiswell, 1994, 2001; Sutton, 2001), which complicates interpretations of water column physics, chemistry and biology.

Suspended particulate matter (SPM) concentrations in the deep-sea typically range from 0.001 to 0.1 mg/l (Eittriem, 1984), with minima at mid-water column depths (typically ~0.005 mg/l) and highest concentrations in the surface mixed-layer and benthic boundary (or nepheloid) layer. Bottom nepheloid layers (BNL) at deep ocean basin sites in the Eastern Pacific and Atlantic oceans (3-6 km deep) have measured SPM concentrations of 0.009-0.3 mg/l (summarised in Eittriem, 1984), which are similar to those from the Chatham Rise area (e.g., McCave and Carter, 1997; Nodder, 1997). The effect of the highly productive Subtropical Front does result in elevated near-surface measurements, compared to these global compilations (e.g., Figure 6.6). Along continental margins, SPM concentrations are generally higher than in the open ocean (e.g., 0.2-1.5 mg/l, Cretan Sea, Chronis et al., 2000; 0.2-10 mg/l, Celtic margin, Atlantic Ocean, Thomsen and van Weering, 1998), such that the single value of 0.3 mg/l observed on the Chatham Rise crest by Nodder (1997) is comparable in magnitude. In this case, however, there is no information on the spatial and temporal dynamics of near-bottom particle populations that can compare to studies conducted on other continental margins. Furthermore, there is no information on the composition and size spectra of SPM on the Chatham Rise.

In terms of sinking particle populations, fluxes in the Chatham Rise region are not substantially different to other measurements from temperate latitudes (e.g., Nodder, 1997; Nodder and Alexander, 1998; Nodder and Northcote, 2001), despite the presence of the highly productive Subtropical Front and the intuitive expectation that high biological production leads to elevated downward fluxes of organic matter. The inference is that considerable recycling of organic matter occurs within the upper water column before material begins to sediment into the deep ocean interior, as reflected by elevated carbon-to- nitrogen (C:N) ratios in sediment trap samples from 300 m water depth on the flanks of the Chatham Rise (Nodder and Northcote, 2001). The sole estimate of a sinking speed for particles in the region of 100 m/d (Nodder and Northcote, 2001) is similar to other inferences elsewhere (e.g., Peterson et al., 2005), with expectations that denser (i.e., foraminifera, mineral grains) and larger particles (i.e., marine snow/aggregates, diatom chains) will have substantially higher sinking rates, dependent on such factors as particle composition, community structure and aggregation/disaggregation rates (e.g., Burd and Jackson, 2009).

Lithogenic material is a natural component in particle flux populations on the Chatham Rise (0.1-0.3 g/m2/y; Nodder and Northcote, 2001), and resuspended material is apparent in near-bottom sediment traps (e.g., mineral grains and glauconite pellets observed in traps deployed 40-70 m above the sea-floor; Nodder and Alexander, 1998). Increases in flux with increasing water depth are ascribed to resuspension and/or lateral advection (Nodder, 1997; Nodder and Alexander, 1998; Nodder and Northcote, 2001), indicating that natural populations of particles in the Chatham Rise area are affected by dynamic vertical and

Natural Sedimentation on the Chatham Rise 23

horizontal processes. Cullen and Singleton (1977) advocated for low current activity and sedimentation rates on central Chatham Rise, but subsequent studies suggest that currents are periodically strong enough to resuspend and transport sediment laterally on the Chatham Rise, perhaps on tidal time-scales (e.g., Nodder et al., 2007).

Lavelle et al. (1981) showed that in a validated modelling study to determine the potential impacts of sediment plumes generated during manganese nodule mining activities in the deep Pacific Ocean, concentrations of SPM in the near-bed plume at 5 m above the seafloor can be in the order of 0.04-0.14 mg/l, compared to ambient concentrations of 0.007 mg/l. The turbidity impact of the mining plume above the seafloor diminished rapidly to such ambient concentrations at 50 m above the seabed. In this study, it was anticipated that the impact of mining activities could extend beyond 100 km from the mining site, with the benthos exposed to elevated turbidity levels for as much as a year.

In comparison, sediment plume modelling for CRP by Hadfield (2011) and Hadfield et al. (2011) shows that the mean flow and eddies within the Subtropical Front, vertical mixing by wind-shear (near-surface, ~100 m) and tides (near-bottom, ~50 m above the seabed), as well as particle sinking velocity, are critical parameters in determining the possible footprint of dispersed sediment on the Chatham Rise. From this initial modelling work, sediment might be dispersed in the order of tens of kilometres from source over periods as low as 10 days (see Figures 11-14 in Hadfield, 2011). 6 Conclusions . There is limited spatial and temporal information on natural sedimentation processes (suspended particulate matter concentrations, sinking fluxes, sedimentation rates) that operate on the Chatham Rise;

. Existing data suggests, however, that, despite the presence of the highly productive Subtropical Front, particulate matter concentrations and fluxes are not dissimilar to other oceanic regions and continental margins at temperate latitudes elsewhere, although elevated concentration and flux values within 50 m of the sea-floor suggest that resuspension and/or lateral advection are prominent processes;

. Surficial sediments are reasonably unconsolidated (e.g., Cullen and Singleton, 1977) and may be resuspended relatively easily and transported by near- bottom currents, perhaps in the order of tens of kilometres away from source on weekly time-scales; and

. In order to better parameterise models of sediment plume and particle dispersal in the Chatham Rise area, additional information is required on the composition, size spectra and the temporal and spatial variability of natural and mined suspended and sinking particulate matter populations. New data acquired by CRP since this report was compiled will go part way to rectifying this situation, although it is expected that longer-term and more spatially intensive data-sets in the vicinity of and in far-field environments of the proposed mining activities will ultimately be required.

24 Natural Sedimentation on the Chatham Rise

7 Acknowledgements Thanks to Ray Wood (GNS Science) and Melissa Bowen (University of Auckland) for provision of the IX Survey mooring data and discussions. Matt Walkington (NIWA) provided the CTD data at short notice, and Helen Neil (NIWA) is thanked for providing a perceptive internal review of the report. 8 References IX Survey (2012). Final Report – Technical note and data availability, Chatham Rise. A report by IX Survey prepared for Chatham Rock Phosphate Ltd. 10p.

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