Accepted Manuscript

Compositional and biogeochemical variations of sediments across the terrestri- al-marine continuum of the Baker-Martínez fjord system (Chile, 48°S)

Lorena Rebolledo, Sebastien Bertrand, Carina B. Lange, Fabian J. Tapia, Eduardo Quiroga, Matthias Troch, Nelson Silva, Paola Cárdenas, Silvio Pantoja

PII: S0079-6611(18)30048-X DOI: https://doi.org/10.1016/j.pocean.2018.12.004 Reference: PROOCE 2045

To appear in: Progress in Oceanography

Received Date: 23 March 2018 Revised Date: 30 November 2018 Accepted Date: 10 December 2018

Please cite this article as: Rebolledo, L., Bertrand, S., Lange, C.B., Tapia, F.J., Quiroga, E., Troch, M., Silva, N., Cárdenas, P., Pantoja, S., Compositional and biogeochemical variations of sediments across the terrestrial-marine continuum of the Baker-Martínez fjord system (Chile, 48°S), Progress in Oceanography (2018), doi: https://doi.org/ 10.1016/j.pocean.2018.12.004

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Compositional and biogeochemical variations of sediments across the terrestrial-marine continuum of the Baker-Martínez fjord system (Chile, 48°S)

Lorena Rebolledoa,b,1, Sebastien Bertrandc,1, Carina B. Langeb,d,e,1, Fabian J. Tapiad,e, Eduardo

Quirogae,f, Matthias Trochc, Nelson Silvaf, Paola Cárdenasb,e,g, Silvio Pantojad,e

a Instituto Antártico Chileno, INACH, Plaza Muñoz Gamero 1055, Punta Arenas, Chile b Centro de Investigación Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Universidad

Austral de Chile, Campus Isla Teja, Valdivia, Chile c Renard Center of Marine Geology, Ghent University, Krijgslaan 281 S8, 9000, Gent, Belgium d Departamento de Oceanografía, Universidad de Concepción, Barrio Universitario, Concepción,

Chile e Centro de Investigación Oceanográfica COPAS Sur-Austral, Universidad de Concepción, Barrio

Universitario, Concepción, Chile f Pontificia Universidad Católica de Valparaíso, Escuela de Ciencias del Mar, Avenida Altamirano

1480, Valparaíso, Chile g Postgrado en Oceanografía, Facultad de Ciencias Naturales y Oceanográficas, Universidad de

Concepción, Barrio Universitario, Concepción, Chile

1 These authors contributed equally to this paper.

Corresponding author: [email protected] ABSTRACT

The Baker-Martínez fjord system (Chile, 48⁰S) is a transitional environment between the terrestrial ecosystems of Patagonia and the SE Pacific Ocean. This unique setting makes it an ideal system to evaluate land-ocean gradients in sediment composition and in a range of biogeochemical variables. Here, we studied the composition and organic geochemistry of surface sediments deposited in the Baker-Martínez fjord system to assess spatial changes in sediment properties and identify the best tracers of terrestrial input. We determined concentrations in biogenic opal, organic carbon, calcium carbonate, and lithogenic particles, and we measured diatom abundance, bulk elemental and isotopic organic geochemistry, and n-alkanes distribution, on a series of surface sediment samples distributed along two proximal-to-distal transects (one in the Martínez

Channel and a second one in the Baker Channel). Results reveal clear proximal-to-distal trends in the proportion of freshwater diatoms and in the fraction of organic carbon of terrestrial origin calculated from the bulk organic δ13C measurements. Diatoms are particularly sensitive to freshwater input in proximal environments, whereas bulk organic geochemistry better reflects terrestrial input over long distances across the fjord system. Other terrestrial proxies frequently used in the literature, such as the N/C ratio and the n-alkane-based terrestrial/aquatic ratio, did not show clear variations with distance from the fjord head. Our results suggest that the isotopic

13 composition of bulk organic matter (δ Corg) and the relative proportion of diatoms of freshwater origin are the best-suited proxies to estimate past changes in terrestrial input from fjord sediment archives, irrespective of the glacial status of the fjord and its watershed.

KEYWORDS: Chilean fjords, sediment composition, organic geochemistry, carbon stable isotopes, n-alkanes, diatoms

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1. Introduction

The Chilean fjords (41–55°S) constitute a dynamic estuarine system characterized by sharp gradients in salinity, aquatic productivity, and suspended sediment concentrations (Aracena et al.,

2011; Sepúlveda et al., 2011; Silva et al. 2011; Vargas et al., 2011). Due to their location at the interface between marine and terrestrial environments, they rapidly respond to changes in continental and marine conditions alike (Sepúlveda et al., 2011; Bertrand et al., 2012b).

High freshwater and detrital inputs from Patagonian rivers and calving glaciers result in a water column that is typically highly stratified with variable amounts of suspended solids (Silva et al., 1998; González et al., 2013; Marín et al., 2013; Quiroga et al., 2016). During the productive season, the fjords can also efficiently export carbon to the sediments, and therefore act as an efficient CO2 sink (Sepúlveda et al., 2011; Torres et al., 2014; Smith et al., 2015). As a result,

Patagonian fjord sediments are increasingly used to reconstruct past changes in river discharge

(Bertrand et al., 2014; Rebolledo et al., 2015), glacier variability (Kilian and Lamy, 2012; Bertrand et al., 2014, 2017), coastal ocean temperature (Sepúlveda et al., 2009; Caniupán et al., 2014), and marine productivity (Rebolledo et al., 2008; Mayr et al., 2014; Aracena et al., 2015).

In recent years, the Baker-Martínez fjord system (48⁰S) has been the focus of several studies, encompassing zooplankton community dynamics and along-fjord changes in estuarine circulation

(Meerhoff et al., 2013, 2014, 2015, 2018); internal tides related to river discharge pulses (Ross et al., 2014); productivity changes in the water column (González et al., 2013); and benthic diversity patterns and trophic structure (Quiroga et al., 2012, 2016). Seasonal changes in river outflow, for example, have been shown to force the along-fjord displacement of salinity and turbidity fronts, influencing plankton and benthic patterns (Meerhoff et al., 2014; Quiroga et al., 2016). Likewise, the warming of subsurface waters along the Martínez Channel in summer and autumn has been linked to intrusions of warmer water from the Gulf of Penas (Aiken, 2012; Moffat et al., 2018).

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Semidiurnal internal tides in the Martínez Channel near the pycnocline were also shown to correlate with high discharge pulses of the Baker River (Ross et al., 2014). In addition, turbidity in

Baker Fjord is increasingly affected by Glacial Lake Outburst Floods (GLOFs; Dussaillant et al., 2010;

Jacquet et al., 2017), which supply high volumes of freshwater with high suspended loads (Marín et al., 2013). So far, very few studies have focused on the spatial patterns in the composition of the sediments deposited along the Baker-Martínez fjord system.

In other Patagonian fjords, sediment carbon stable isotopes have provided reliable information about the sources of organic matter (e.g., Sepúlveda et al., 2011; Silva et al., 2011;

Vargas et al., 2011; Lafon et al., 2014; Mayr et al., 2014), and long-chain n-alkanes have been used as proxies for the provenance of terrigenous inputs (Pinto and Bonert, 2005; Sepúlveda et al.,

2011; Placencia et al., 2017). Additionally, biogenic opal and marine diatoms preserved in fjord sediments have proven to be reliable proxies of changes in siliceous phytoplankton export production (e.g., Rebolledo et al., 2005, 2015; Sepúlveda et al., 2005; Aracena et al., 2011), and increases in the contents of freshwater diatoms in sediments have been related to periods of high precipitation and river discharge (Rebolledo et al., 2015).

With this in mind, the main goal of this study was to evaluate the spatial distribution of sedimentological and biogeochemical proxies, as well as diatom abundance and species composition, of surface sediments from the Baker-Martinez fjord system to test their application for paleoclimate and paleoenvironmental reconstructions in a region that appears to be particularly sensitive to climate change. Since the Baker-Martinez fjord system is one of the few

Patagonian systems directly open to the Pacific Ocean, it has the advantage of covering the entire continuum of terrestrial to marine conditions, making our results applicable to investigate sediments deposited under different environmental conditions in the past.

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2. Study area

The Baker-Martinez fjord system is a ~150 km long, W-E oriented basin that connects the glaciated Patagonian Andes to the east with the SE Pacific Ocean to the west through the Gulf of

Penas (Fig. 1). It is located between the Northern and Southern Patagonian Icefields (NPI and SPI) and it comprises two main channels: Martínez to the north and Baker to the south (Fig. 1). The fjord system has a complex bathymetry with shallow depths (<50 m) at the head near the town of

Tortel and deep basins of more than 1000 m in the middle of Baker Channel (Fig. 2, top panel).

This system formed during the last deglaciation when the ice masses extended to the Pacific

Ocean and it is thought to have reached its present-day configuration after the separation of the

Patagonian Ice Sheet into the NPI and SPI approximately 8000 years ago (Glasser et al., 2016).

The Baker-Martinez fjord system is almost entirely fed by two large rivers: Baker (mean discharge ~1100 m3/s) and Pascua (~750 m3/s), which originate from the large lakes General

Carrera and O’Higgins, respectively (Fig. 1). The discharge of Baker and Pascua rivers is highest in austral summer (December–March), i.e., during the snowmelt season, and lowest in winter. This hydrological regime may be defined as nival with glacial contributions (e.g., Lara et al., 2015).

Additionally, the channels receive freshwater from Steffen Fjord, which is fed by meltwaters from

Steffen Glacier (the third largest glacier of the NPI; Willis et al., 2012) via Huemules River, and from the calving Jorge Montt Glacier (the northernmost SPI glacier) (Fig. 1). The latter has experienced a rapid retreat during the last decade, with a rate of almost a kilometer per year

(Rivera et al., 2012).

Circulation studies based on ADCP deployments and CTD measurements have shown that the circulation in the area corresponds to a typical two-layer estuarine system, with maximum surface velocities that can reach up to 20 cm s-1 in summer (Cáceres, 2007; Ross et al., 2014, 2015). Tides are mixed, mainly semidiurnal with amplitudes of 1 m (Ross et al., 2015), and wind velocities are

5 controlled year-round by the strength of the southern westerly winds (SWW). Modeling studies have shown that regional wind forcing is a key factor explaining the circulation and hydrography of the Gulf of Penas and Baker Channel (Aiken, 2012), including the modulation of warm water intrusions into proglacial fjord Jorge Montt (Moffat, 2014; Moffat et al., 2018).

Although data on primary productivity in this region are scarce, they reveal low values of 91 mg C m-2 d-1 near Caleta Tortel (Aracena et al., 2011) and 650 mg C m-2 d-1 in Steffen Fjord (Jacob et al., 2014). Pico- and nano-plankton dominates at inner sites, whereas a dominance of chain- forming diatom genera (i.e., Thalassiosira, Chaetoceros, Skeletonema) is observed in outer, more oceanic sections of the fjord. During spring, high loads of suspended sediments (42−105 mg/L in the Martínez Channel; Marín et al., 2013) dramatically increase the surface attenuation of incident light, which may limit primary productivity (e.g., González et al., 2013; Quiroga et al., 2016).

The region’s climate is maritime hyperhumid, with highest precipitation between May and

August (late fall and winter), and a mean air temperature of 8.9°C (Hijmans et al., 2005).

Vegetation cover in the Baker and Pascua river basins consists of evergreen and deciduous native forests, shrublands, and peatlands (Luebert and Pliscoff, 2006; Rodríguez et al., 2008). Barren land is located above the treeline. All prominent species use the C3 photosynthetic pathway (Powell and Still, 2009). In the vicinity of Caleta Tortel, located next to the Baker River mouth, the vegetation is dominated by Nothofagus, shrubs, and ferns. Anthropogenic disturbances consist of logging by rural landowners, overgrazing by livestock, and road construction, which are promoting biological invasions in the forests of the Baker River basin (Vidal et al., 2011).

3. Material and methods

3.1 Hydrographic data collection

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Sixteen vertical CTD profiles were acquired in the Baker-Martinez fjord system (Fig. 2) during the COPAS Sur-Austral 2014 campaign (October 2014), using a Seabird SBE model 25 CTD with a sampling frequency of 8 Hz and average descent rate of 1 m s-1. The CTD profiles included temperature, salinity, oxygen concentration, turbidity and fluorescence, down to a maximum of

447 m (station 14; Fig. 2). The same stations were revisited in October 2015 (spring), and January and March 2016 (summer) to capture temporal variability in the spatial structure of hydrographic variables during the productive season. The October 2015 survey was conducted using a different

CTD (RBR XR-620), with a sampling rate of 6 Hz. During post-processing, all CTD data were depth- binned to obtain profiles with a vertical resolution of 1 m (i.e. 1 dbar).

To characterize temporal variability in salinity along the Martínez Channel, we used a set of sub-surface salinity records gathered over one year (October 2012 through November 2013) at 5 points along the Martinez Channel. At each station, conductivity-temperature loggers (HOBO U24,

Onset Computer, USA) were deployed at depths that ranged between 1 and 2 m below mean lower low water (MLLW). Salinity time series were obtained from conductivity data based on calibration factors provided by the manufacturer. For each time step in the salinity record along the Martinez Channel, linear interpolation was used to determine the along-fjord position of the

10 and 20 salinity contours. These values were later used to characterize the seasonal variability in the position and strength of the surface salinity front.

3.2 Sampling

During the October 2014 campaign, water samples were collected at 11 stations with Niskin bottles to measure chlorophyll-a (Chl-a) concentrations down to 30, 40, or 50 m depth, depending on location (Table S1). Duplicate samples were frozen at -20° C prior to analysis by fluorometry

(Parsons et al., 1984). Chl-a values were integrated over the upper 30 m of the water column.

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Vertical net tows (25 µm mesh) were used to collect phytoplankton samples from 0–15 m for qualitative analysis at stations CO14-E3, E4, E5, E6, E8, and E13 (Fig. 1; Table S1). Observation of phytoplankton samples was done at 400X magnification using a Zeiss Axio Scope A1 microscope with phase contrast.

Surface sediment samples were collected during several campaigns carried out in the Baker-

Martinez fjord system and in Jorge Montt Fjord between 2008 and 2014: CIMAR 14 Fiordos

(October-November 2008; CF14), Jorge Montt 2013 (March and November 2013; Gu13), Martínez

Channel 2014 (March and August 2014; Qu14; Quiroga et al., 2016), COPAS Sur-Austral 2014

(October 2014; CO14), and CIMAR 20 Fiordos (November 2014; CF20) (Fig. 1; Table S2). A total of

53 surface sediment samples were obtained using box and Rumhor corers (0–1 cm) and grab samplers. For each sampling location, we measured the distance from the nearest river or calving glacier front using Google Earth (paths running through the center of the fjords). The spatial distribution of the sedimentological and geochemical variables measured in the surface sediments

(Table S2) was visualized on maps made using Ocean Data View 4.7.10 (Schlitzer, 2017) and Global

Mapper 16.

In addition, living vegetation was collected near Caleta Tortel during the COPAS Sur-Austral

2014 campaign with the main goal of defining the terrestrial end-member of geochemical parameters to be compared with the signal preserved in the surface sediments. This terrestrial end-member was additionally constrained by a bulk river sediment sample (RS11-05) collected 8 km upstream of the outflow of Baker River.

3.3 Sediment composition

Fjord sediments are generally composed of variable mixtures of lithogenic particles, organic matter, biogenic opal, and carbonates (Bertrand et al., 2012b). The relative contents of these four

8 sediment components were quantified according to the following methods.

Biogenic opal was estimated following the alkaline extraction procedure described by

Mortlock and Froelich (1989) and modified by Müller and Schneider (1993), which consists in extracting 50 mg of sediment with NaOH at 85°C for 5 hours. Extraction and analysis by molybdate-blue spectrophotometry were conducted at the laboratories of Marine Organic

Geochemistry and Paleoceanography, University of Concepción, Chile. Values are expressed as biogenic opal after multiplication of the Si (%) values by 2.4 (Mortlock and Froelich, 1989).

Analytical precision was ± 0.5%.

The weight percentage of total inorganic carbon (TIC) was determined using an UIC CM5017 coulometer equipped with a CM5330 acidification module at Ghent University, Belgium. For each sample, approximately 50 mg of sediment was precisely weighed into a glass flask and treated with 5 ml 2N H3PO4 to release CO2. This method assumes that 100% of the measured CO2 is derived from the dissolution of calcium carbonate. Calcium carbonate concentrations were calculated as CaCO3 = TIC × 8.33. The detection limit was 0.03% CaCO3.

The bulk organic geochemistry of the sediments was measured in two different laboratories:

Bioanalytical laboratory, School of Biological Sciences, Washington State University, USA (most samples), and Laboratory of Biogeochemistry and Applied Stable Isotopes (LABASI-PUC), Chile (the

11 samples from the CF14 cruise, and samples Qu14-E1Q and Qu14-E4Q). To determine organic carbon (Corg) and total nitrogen (Ntot) contents, freeze-dried sediments were ground and acidified

(with 2N phosphoric acid at Bioanalytical laboratory and with HCl 2:1 v/v at LABASI) to remove inorganic carbon. Corg and Ntot measurements were made on a Eurovector (Milan, Italy) elemental analyzer at Bioanalytical laboratory, and on a Flash EA2000 elemental analyser at LABASI. Organic matter (OM) was calculated from the Corg results as OM = Corg × 2.2 (Bertrand et al., 2012b).

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The content of lithogenic particles (LP) was calculated as: LP = 100 % - CaCO3 - 2.2 × Corg - biogenic opal, following Bertrand et al. (2012b).

3.4 Bulk organic geochemistry

13 15 Carbon (δ Corg) and nitrogen (δ N) stable isotopes were measured at the two laboratories mentioned above. Most samples were measured at Bioanalytical laboratory using an elemental analyzer coupled with an Isoprime isotope ratio mass spectrometer (IRMS), with a precision

13 15 ±0.1‰ for δ Corg and 0.3‰ for δ N. The running standard was a protein hydrolysate calibrated against NIST standards. At LABASI, a Thermo Delta Advantage IRMS coupled with the Flash EA

2000 elemental analyser was used. Acetanilide, atropine, caffeine, glutamic acid, and high organic

13 15 sediment were used as internal standards. δ Corg and δ N were determined with a precision of

±0.18 and 0.2 ‰, respectively.

The vegetation samples collected near Caleta Tortel were analysed for Corg and Ntot concentrations and stable isotopic composition at Bioanalytical laboratory, USA (Table S3). Prior to analysis, samples were washed with distilled water, freeze-dried, and ground. About 1 mg of plant tissue was put into a tin capsule and acidified with 2N phosphoric acid before analysis.

The <90 µm fraction of river sediment sample RS11-05 was isolated by dry sieving, and its bulk organic geochemical composition was analyzed at the UCDavis Stable Isotope Facility after acidification of the sample with 1N sulfurous acid. The precision, calculated by replicate analysis of

13 15 an internal standard, was 0.05‰ for δ Corg and 0.08‰ for δ N.

13 We used the δ Corg of the sediment samples to estimate the relative contributions of terrestrial and aquatic organic carbon (Meyers, 1997; Perdue and Koprivnjak, 2007; Sepúlveda et al., 2011). The fraction of terrestrial Corg (fT) was calculated using a two end-member (linear) mixing model (Thornton and McManus, 1994):

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– (1) –

13 13 13 where δ Corg sample, δ Corg marine, and δ Corg terr are the isotopic values of the sample, terrestrial end-member, and marine end-member, respectively. The end-member values used in this study were determined on samples collected in fully marine and fully terrestrial conditions: -19.86‰ for the marine end-member (site GeoB3323-4 on the continental margin off the coast of Northern

Chilean Patagonia; Hebbeln et al., 2000) and -27.72 ‰ for the terrestrial end-member ( average of five <125 µm sediment samples from rivers that discharge into the fjords; Bertrand et al., 2012b).

The latter value is supported by the δ13C composition of sediment sample RS11-05 <90 µm from

Baker River (-26.74‰). The proportion of marine organic carbon (ƒM) was calculated by difference

(100 % - ƒT). For three samples, the application of Eq. (1) provided ƒT values above 100, which were then modeled to 100.

3.5 n-alkanes

Extraction of lipid biomarkers from sediments was carried out according to a modified Bligh and Dyer (1959) protocol. Briefly, 0.3–2 g of freeze-dried samples (surface sediments and plant tissues) were extracted by accelerated solvent extraction (Dionex ASE350) at high pressure (1500 psi) and high temperature (100 °C) with a mixture of dichloromethane and methanol (DCM:MeOH,

3:1 v/v). Column chromatography was applied to the total lipid extract using ca. 1g of silica gel as stationary phase and successively 3/8 DV (Dead Volume) Hexane, 2 DV (DCM), 2DV DCM/EtoAC

(1:1) and 2DV DCM/MeOH (7:3) to elute the fractions containing hydrocarbons (F1), esters and ketones (F2), sterols and alcohols (F3) and fatty acids (F4).

Samples for n-alkanes were analyzed on an Agilent 6890 GC series coupled to an Agilent 5972 mass spectrometer (MS), at the laboratory of Marine Organic Geochemistry, University of

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Concepción, Chile. The chromatograph was equipped with a fused silica capillary column (30 m length, i.d. = 0.25 mm HP5-MS, film thickness = 0.25 µm) and an automated injection system. The

GC oven temperature program was 80 °C to 130 °C at 20 °C min-1, and then to 310 °C at 4 °C min-1.

The MS was operated in electron impact mode (70 eV).

Long chain n-alkanes are important components of leaf waxes (Bianchi and Canuel, 2011). In higher plants, n-alkanes show a predominance of molecules containing between 27 and 35 carbon atoms, with a prevalence of odd- over even-numbered carbon chains (Eglinton and Hamilton,

1967). Aquatic sources such as algae and photosynthetic bacteria are dominated by C15, C17, and

C19. The shorter chain n-alkanes (C14−C25) are therefore mainly of algal phytoplankton origin (e.g.,

Eglinton and Hamilton 1967; Liu and Liu, 2016).

Three n-alkane-based indices were calculated as follows:

(1) The average chain length (ACL), i.e., the weight-averaged number of carbon atoms of

the higher plant C23-35 n-alkane, was calculated following Poynter et al. (1989):

ACL = (2) 23-35

(2) The Carbon Preference Index (CPI) was calculated following Allan and Douglas (1977):

CPI = (3)

(3) The terrigenous/aquatic ratio (TAR), which provides an estimate of the algal vs.

terrestrial organic matter, was calculated following Bourbonniere and Meyers (1996):

TAR = (4)

ACL mostly reflects vegetation type (El Nemr et al., 2016), but it has also been shown to reflect temperature, humidity, carbon assimilation pathway (C3 vs. C4), and the woody vs. non-woody

12 nature of the plants (Bush and McInerney, 2015). In paleo-studies, ACL is generally used as a climate proxy, based on the observation that, on average, plants in forests form shorter chains than grassland leaves (Cranwell, 1973). Likewise, in warmer climates, plants tend to produce longer chains of n-alkanes (Jeng, 2006).

CPI and TAR have been used to estimate the source of OM in sediments. CPI values >5 are indicative of n-alkanes derived from terrestrial sources, i.e., land-plants, which have a predominance of odd-numbered carbon chains (Eglinton and Hamilton, 1963; Hedges and Prahl,

1993), whereas values 1.0–1.2 and < 1.0 reflect marine plankton (Corner, 1979) and petrogenic inputs (Pendoley, 1992), respectively. However, it is worth noting that marine plankton does not affect bulk CPI values much since plankton mostly produces short-chain n-alkanes (Rosell-Melé and McClymont, 2007). In addition, CPI may reflect the state of degradation of organic matter because the odd-even preference tends to decrease with diagenesis (Bianchi and Canuel, 2011).

TAR values reflect the ratio of vascular land plants over aquatic sources (Bourbonniere and

Meyers, 1996; Meyers, 1997). The absolute values may however be affected by selective biodegradation (Peters et al., 2007).

3.6 Siliceous microorganisms

For siliceous microorganisms (mainly diatoms but also including sponge spicules, silicoflagellates, and chrysophyte cysts), 0.5 g of freeze-dried sediment of each sample was oxidized using a mix of 30% hydrogen peroxide, sodium pyrophosphate, and hydrochloric acid to remove organic matter, clays, and carbonates, respectively. 0.2 ml of clean sample was deposited onto a cover slip, air-dried, and mounted with Naphrax mounting medium (refraction index = 1.7).

Two permanent slides per sample were prepared in this fashion and observed under a Zeiss Axio

Scope A1 microscope with phase contrast at 400× and 1000×. Siliceous microorganisms were

13 counted on transects on microslides covering 1/2 or 1/5 of the slide, following counting procedures of Schrader and Gersonde (1978). Concentrations are expressed as numbers per gram dry weight (i.e., diatom valves g-1). Diatoms were determined to the lowest taxonomic level following Cupp (1943), Rivera (1981), Hasle and Syvrtsen (1996), Sims (1996), and Witkowski et al.

(2000). Diatoms were grouped by ecological affinity into the following categories (based on Cupp

(1943), Round et al. (1990), Sims (1996), Witkowski et al. (2000), and Romero and Hebbeln (2003);

Table S4): high nutrient (HN), coastal planktonic (CP), warm water (WW), cold water (CW), marine benthic (MB), and freshwater (FW). Laboratory and microscopy work was carried out at the

Paleoceanography laboratory, University of Concepción, Chile.

3.7 Statistical analysis

To identify the combination of variables that are most influential in terms of hydrographic structure along the fjord, Principal Component Analysis (PCA) was applied to 0–20 m depth averages of the four main variables (temperature, salinity, turbidity, and fluorescence) measured during the CTD surveys of October 2014, October 2015, January 2016, and March 2016. A second

PCA (Spearman; XLStat 2016.01) was performed on the following 14 sediment variables: distance from the nearest river mouth/calving glacier front, biogenic opal, Corg, CaCO3, lithogenic particles

13 15 (LP), N/C molar ratio, δ Corg, δ N, total diatoms, total n-alkanes, ACL, CPI, TAR, and the proportion of freshwater diatoms. Spearman PCA is preferred over Pearson PCA because

Spearman evaluates the monotonic, instead of linear, relationship between variables. The same software was used to calculate Pearson correlation coefficients and p-values (two-tailed test of significance) between variables. Correlations at p < 0.01 were considered significant.

4. Results

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4.1 Water column

Surface water temperature in the Baker-Martinez fjord system in October 2014 ranged from

3.9 to 10.4°C, with a progressive warming towards the outer section of the channels (Fig. 2). The warmest waters were found at ~50 m depth. The lowest salinity values (< 3 PSU) were observed within the upper water column (0–5 m) at inner-fjord stations of the Baker-Martinez fjord system, i.e., close to the mouths of Baker and Pascua rivers (Fig. 2). This low-salinity layer became thinner towards the west, where it reached less than 3 m. Overall, salinity fluctuated widely between 1 and 33.4 PSU, with the highest values occurring in the deepest waters. Dissolved oxygen ranged from 8.2 mL/L in surface waters to 3.2 mL/L at depth. No anoxic/hypoxic waters were detected during the survey. Turbidity was higher in the upper water column, with the highest values occurring in the colder surface water, especially near river mouths and calving glaciers. Turbidity was lower at the outer fjord stations, where it fluctuated between 1.0 and 24.6 NTU. Surface turbidity was 10x higher at inner and glacier sites than at more oceanic sites (Fig. 2). Fluorescence, which is often used as a proxy for phytoplankton biomass, was essentially limited to the upper 20–

25 m of the water column, and showed a clear increase towards the outer part of the fjord system, especially in Baker Channel (Fig. 2).

The PCA of the CTD data (Fig. 3) showed high positive covariance between fluorescence and salinity, which covaried negatively with turbidity. Our analysis also indicated that fluorescence and temperature were practically independent.

Concentrations of Chl-a integrated over the upper 30 m of the water column ranged between

8.3–92.5 mg Chl-a m-2 in the Martínez Channel and 11.5‒114.3 mg Chl-a m-2 in the Baker Channel

(Fig. 4a; Table S1). In both channels, the lowest values occurred in the most proximal environments.

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Twenty-five diatom taxa were recorded in the net tows of the six stations that were sampled.

Among them, chains of the genera Chaetoceros, Pseudo-nitzschia, Skeletonema, and Cerataulina pelagica, and colonies of Thalassionema nitzschioides were commonly observed. Inner fjord station E13 was the least diverse in terms of the numbers of diatom taxa (only 7 taxa were observed; Table S5).

4.2 Bulk sediment composition

Throughout the Baker-Martinez fjord system, organic matter contents varied between 0.35 and 9.09 wt. %, biogenic opal contents ranged from 1.1 to 10.2 wt. %, and CaCO3 concentrations were always below 0.3 wt. %, except at some outer fjord locations where they reached up to 2.4 wt. % (Fig. 5a-c; Table S2). The highest contents in organic matter, biogenic opal and CaCO3 all occurred toward the outer section of the fjord system. In contrast, the lithogenic particle concentrations were high along the entire fjord system (84.8–97.2 wt. %), with peak values occurring near the Baker River mouth and Steffen Fjord (Fig. 5d). Station CO14-E6 in the outer section of Martinez Channel (Fig. 1) contained high amounts of mollusk fragments, which explains the particularly high CaCO3 concentrations (16.9%; Table S2). This station was therefore considered as an outlier.

4.3 Bulk organic geochemistry

Corg is generally low along Martínez and Baker channels (<0.75%), with the lowest value

(0.16%) being recorded at station Gu13-E2, near Jorge Montt Glacier. Higher values (>1%) were only reached towards the marine end of the fjord system (Fig. 6a). An exceptionally high value of

4.4% was observed at shallow station Qu14-E4Q (Fig. 6a). In general, the N/C values fell within the range 0.08–0.20, with lower values at inner-fjord sites and near river mouths than towards the

16

open coast (Fig. 6b). Significantly higher N/C values (average: 0.2–0.5) were however observed in

glacial fjord Jorge Montt sediments, where samples have extremely low Corg contents (< 0.3 %;

Table S2; Figs. 6a, b).

13 Sedimentary stable isotopes of organic carbon (δ Corg) showed depleted values (-25 to -29‰)

at mid- and inner fjord locations (e.g., heads of Baker and Martinez channels, and proglacial fjord

Jorge Montt) compared to sediments toward the outer fjords and in the Gulf of Penas (-21 to -

23‰, Fig. 6c). The proportion of terrestrial organic carbon varied between 15 and 100% and

13 obviously followed the same pattern as δ Corg (Fig. 6d).

4.4 n-alkanes

We compared the n-alkane composition of sediment samples from the Baker-Martinez fjord

system with living vegetation recovered near Caleta Tortel (Fig. 7, Table S3). n-alkanes of chain

length between C16 and C37 were recorded from trees, shrubs, and fern leaves (n=8) with total

concentrations varying from 1246 to 6073 µg g dw-1 (Figs. 7a and S1; Table S3). A marked

dominance of odd- over even-numbered n-alkanes was observed (Figs. 7a and S1). In sediments,

total n-alkane concentrations varied from 0.9 to 50.5 µg g dw-1 (Fig. 7b, Table S2) with chain

lengths between n-C14 and n-C38 (n=37) and a prevalence of odd- over even-numbered carbon

chains (Fig. 7b).

When comparing results from living vegetation with surface sediments (Fig. 7), it is evident that

the total concentration of n-alkanes preserved in the sediments is ~3 orders of magnitude lower

than for living vascular plants, which is consistent with the low sediment TOC values. Although

both exhibit a similar pattern in the odd- over even-numbered carbon prevalence, the pattern was

better expressed in the living vegetation.

17

For sediments, the ACL23–35 ranged from 27.9 to 29.5, with no clear spatial trend, although the three lowest values were all located in the central part of the Baker-Martínez fjord system (Fig. 8a;

Table S2). The values for modern vegetation were similar (27.7–29.5), except for the shrub

Berberis ilicifolia (26.9) (Fig. S1, Table S3). CPI in surface sediments varied between 1.0 and 7.5

(Fig. 8b; Table S2), indicating a predominantly terrestrial source, and the values were positively correlated with ACL (r=0.58; p< 0.001). For the vegetation samples, CPI ranged from 9.1−46.3, with the most extreme value (46.3) being again associated with B. ilicifolia (Fig. S1, Table S3). TAR values were high and particularly variable throughout the study area. They ranged from 2.7 to 18.7 in the middle and outer sections of the Baker-Martínez fjord system, and from 3.1 to 132.1 near rivers and in glacial fjords (Fig. 8c, Table S2).

4.5 Siliceous microfossils

The analysis of siliceous microfossils in the surface sediment samples (Table S2) revealed a dominance of diatoms, composing >99 % of the total siliceous assemblages. Sponge spicules, silicoflagellates, and phytoliths were rare. The lowest diatom abundances were observed in glacial fjord Jorge Montt (4 × 103 to 1.2 × 104 valves gdw-1), in contrast to outer fjord stations where peak values were recorded (2 × 107 valves gdw-1 at station Qu14-E3Q) (Table S2). There was a significant positive correlation between diatom abundance and biogenic opal values (r= 0.64, p<0.001).

Resting spores of the genus Chaetoceros (C. radicans/cinctus, C. coronatus, C. didymus, C. diadema, C. debilis) accompanied by Thalassionema nitzschioides and Skeletonema japonicum, represented almost 50% of the total diatom assemblage. Other diatom groups were also recorded: coastal planktonic (Thalassiosira eccentrica, T. oestrupii, T. angulata, Rhizosolenia setigera f. pungens); tycopelagic and benthic (Paralia sulcata, Cocconeis costata, Cocconeis pacifica); and freshwater (e.g., resting spores of Chaetoceros muellerii, and several species of the genera

18

Tabularia, Fragilaria, Frustulia, Cymbella, Navicula, and Synedra) (Table S4). The contribution of the freshwater group to the total diatom assemblage fluctuated from 3.2 to 100%; it was the most abundant group in Jorge Montt fjord sediments and at stations close to the Baker and Pascua rivers (Fig. 8d, Table S5). Low values were observed at outer sites.

4.6 Principal component analysis of sediment variables

The PCA of the 14 sediment variables showed that the first two principal components explain

68 % of the total variance (Fig. 9). PC1 accounts for 51% of the total variance and the variables with the highest F1 loadings are diatom abundance (0.96), proportion of freshwater diatoms (-

0.91), δ13C (0.87), distance from the nearest river mouth/calving glacier front (0.80), and proportion of lithogenic particles (-0.80).

5. Discussion

5.1 Spatial and temporal variations in fjord productivity

Depth-integrated (0−30 m) Chl-a values in spring 2014 (October) ranged from 8 to 114 mg Chl- a m-2 (Fig. 4a; Table S1) with average values of 42 mg m-2 in Martínez Channel and 49 mg m-2 in

Baker Channel. The low depth-integrated values at COPAS station CO14-E1 near Tortel and at the mouth of glacial fjord Steffen are comparable to those measured by Jacob et al. (2014) in the same area. Similarly low values were observed at the head of Baker Channel, near the mouth of Pascua

River (Fig. 4a). In both cases, these low values correspond to sites with particularly low surface salinities and with the highest suspended sediment concentrations (Fig. 2), in agreement with

González et al. (2013) and Quiroga et al. (2016), suggesting that primary production in the Baker-

Martínez system is limited by low light availability due to abundant suspended sediments in the water column. This is also in agreement with the regional-scale results of Jacob et al. (2014), who

19 suggested that, in addition to low photosynthetically available radiation south of 46°S, low light availability due to suspended detritus in the water column − particularly at sites with high freshwater inputs of glacial origin − might limit primary production to the upper 15 m of the water column.

In contrast to the inner-fjord sites, Chl-a estimates were particularly high at the outermost sites during our spring 2014 survey (Fig. 4a, Table S1), in agreement with the summer 2015

(January) observations of Quiroga et al. (2016). This however contrasts with the pattern observed by Quiroga et al. (2016) in winter 2014 (August), when Chl-a concentrations were overall lower, and when the highest values occurred at mid-fjord locations in the Martínez Channel.

The clear covariance between fluorescence, which is commonly used as a proxy for phytoplankton biomass, and salinity in the upper 20m of the water column over a 3-year period

(Fig. 3) suggests that salinity can be used to estimate the position of the productivity front across the Baker-Martínez fjord system. This is in agreement with both published and unpublished hydrographic observations for the study area (Meerhoff et al. 2014, Quiroga et al. 2016, Tapia et al., submitted), which point to a correspondence between the surface salinity front – with salinities changing rapidly from <10 to >20 PSU – and maximum chlorophyll-a values, which in all likelihood correspond with areas of greater carbon fluxes from the water column to the fjord sediments. In Martínez Channel (Fig. 4b), low salinities reached their westernmost position in

December-February (austral summer), which are the months of maximum freshwater discharge from the Baker river. In fact, the distance between median positions of the 10 and 20 salinity contours in January-February (Fig. 4b) point to a sharp surface salinity gradient in these months, due to the high freshwater discharge associated with glacier melting. The other time of the year when these two contours are close to each other is the month of October, when river influence reaches its lowest point – due to minimal freshwater discharge in early spring – and marine

20 conditions are most influential (Fig. 4b). Considering that chlorophyll-a tends to significantly increase seaward of the 10–20 PSU salinity front, the fjord section where productivity increases significantly should be located between stations 4 and 5 through fall-winter, and farther out

(between stations 5 and 6) in the late spring and summer months (Fig. 4b).

5.2 Water column–surface sediments comparison

The spring-summer pattern of “inner fjord: low Chl-a – outer fjord: high Chl-a” was also reflected in the Chl-a values measured in surface sediments by Quiroga et al., 2016. Interestingly, our diatom abundance estimates in the sediments of the Baker-Martinez fjord system seem to reflect the same pattern (Table S2).

The qualitative diatom analysis of phytoplankton net tows (0‒15 m) yielded 25 taxa (Table S5) that are common members of the spring micro-phytoplankton assemblage in Patagonian fjords and channels between ~47°S and 50°S (González et al., 2013). However, there is a striking difference in the composition of the diatom assemblage present in the water column with the one preserved in the sediments (Table S5), which was dominated by the robust resting spores of the genus Chaetoceros. In contrast, the fragile frustules of i.e., Pseudo-nitzschia, Cerataulina, and

Chaetoceros vegetative cells were a minor or missing component of the surface sediments.

Unfortunately, we do not have measurements in the area to be able to discern among the various factors (grazing, bottom currents, erosion, lateral advection; e.g., Sancetta and Calvert, 1988;

Muñoz and Wellner, 2016) that may have affected and modified the assemblages either in the water column or at the water/sediment interface.

Freshwater diatoms were only recorded in sediments (ca. 30% of total assemblage on average), mostly at sites near rivers and glacier fronts (Fig. 8d). We did not observe any freshwater diatoms in the water column (Table S5), suggesting that freshwater species sink rather rapidly

21 after being discharged by rivers, and/or that the river supply of freshwater diatoms is seasonal (no significant production of freshwater diatoms in spring). Because freshwater diatoms are washed into the fjord environment from the continent (mainly by rivers), they should however constitute a good proxy to reconstruct changes in terrestrial input, as suggested by the significant negative covariance between the proportion of freshwater diatoms and distance from river mouths/calving glacier fronts (Fig. 9).

5.3 Variations in sediment composition

Our biogenic opal values (1.1–10.2%; Fig. 5) extend the range of data previously given by

Aracena et al. (2011) for our study area and are in agreement with the ones reported by Silva et al.

(2009) and Bertrand et al. (2012b) for the fjords of northern Chilean Patagonia (1.4−12.6% and

1.9–17.7%, respectively). Spatial distribution of Corg in the surface sediments follows that of biogenic opal (r=0.72; p<0.001) and diatoms (r=0.50; p<0.01), with the lowest values observed at stations near glacial fjords and the highest values occurring in the seaward section of the Baker and Martínez channels (Fig. 5; Table S2). Carbonate contents are about an order of magnitude lower than opal values, providing evidence that the main producers in the Baker-Martinez fjord system are silica-based organisms.

The similarity of trends in Corg, biogenic opal, and, to a lesser extent, carbonate contents seems to largely reflect dilution by lithogenic particles, which are supplied by rivers and calving glaciers.

This is also clearly illustrated in Figure 9, where biogenic opal, carbonate and Corg concentrations have clear positive PC1 loadings, whereas lithogenic particles are clearly negatively loaded.

Comparing biogenic silica and carbonate productivity between sites would therefore require measuring accumulation rates at each site, which is beyond the scope of this study. These results

22 also imply that identifying variables of terrestrial input and productivity should focus on indicators that are independent of accumulation rates.

5.4 Sources of sedimentary organic matter

The sediments of the Baker-Martinez fjord system are characterized by variable amounts of terrestrial and marine organic matter. In sedimentary environments, bulk N/C and δ13C have been equally used to estimate the proportions of terrestrial and marine organic matter since terrestrial vegetation is characterized by lower N/C and δ13C than marine organic matter (Meyers, 1997;

Perdue and Koprivnjak, 2007). Both approaches have been successfully applied to modern and

Holocene sediments from the Chilean fjords (e.g., Sepúlveda et al., 2009, 2011; Rebolledo et al.,

2008; Silva et al., 2011; Bertrand et al., 2012a, 2012b, 2014, 2017; Lafon et al., 2014), for which regional end-member values have been defined (terrestrial N/C = 0.0624, δ13C = -27.72‰, and marine N/C = 0.130, δ13C = -19.86‰; Sepúlveda et al., 2011; Bertrand et al., 2012b). In the Baker-

Martinez system, both variables decrease towards the open ocean (Fig. 6b, c; see also the lower density results of Quiroga et al., 2016), as they do for fjords in northern Patagonia (Silva et al.,

2011).

N/C values measured in the sediments of proglacial fjord Jorge Montt and in the prodeltas of

Pascua and Baker rivers are much higher than expected for proximal environments (Fig. 10). This is likely due to the combination of very low TOC (~0.2%) in proglacial environments and the abundance of inorganic nitrogen bound to glacial clays (Kumar et al., 2016), resulting in particularly high N/C values that cannot be directly used to infer organic matter sources.

Additionally, the relatively high abundance of cyanobacteria (mostly Synechococcus) in Jorge

Montt Fjord (Gutiérrez et al., 2015) may contribute to the high N/C ratio since Synechococcus is known to efficiently fix nitrogen in proglacial environments (Zhu et al., 2016). This contrasts with

23 sites that are not directly affected by glacial clay input, where N/C and δ13C are positively correlated (Fig. 10), although glacial clays likely influence N/C values throughout the entire fjord system since Baker and Pascua rivers, as well as Jorge Montt Glacier, are the main sources of sediment to this fjord system. In the (non-glacial) fjords further to the north, the fraction of terrestrial carbon calculated from N/C and δ13C is highly correlated (r=0.94, p < 0.001; Bertrand et al., 2012b). These observations suggest that δ13C is better suited than N/C to calculate the fraction of terrestrial carbon in fjord sediments, as it is independent of the fjord’s glacial status.

The TAR ratio for all studied locations was >>1, which can be interpreted as a predominantly terrestrial origin of the n-alkanes throughout the Baker-Martínez fjord system (Bourbonniere and

Meyers, 1996), in agreement with the observation that the highest TAR values are found close to rivers and in glacial fjords (Fig. 8c). This interpretation of the TAR values is, however, in contradiction with the quantification of terrestrial vs. aquatic sources of organic carbon based on

δ13C (Fig. 6d), which shows that terrestrial organic carbon only dominates in the most proximal environments. The likely reason behind this discrepancy is that land-plant organic matter typically contains more n-alkanes than marine organic matter, resulting in a disproportionate weight assigned to land-plant input (Peters et al., 2007). This bias is also likely responsible for the lack of significant correlation between TAR and δ13C (r=-0.347, p=0.028).

The ACL values measured in our surface sediment samples all fell between 27.9 and 29.5, and the three lowest values occurred in the central part of the Baker-Martínez fjord system (Fig. 8a).

The lack of a significant relation with distance from river mouths/calving glacier fronts (Fig. 9) reflects the relatively homogenous nature of the evergreen vegetation along the fjord system

(Luebert and Pliscoff, 2006), and the absence of significant tributaries in the western and central part of the fjord system, which is also the case for fjords further north (Sepúlveda et al., 2011). The presence of the lowest ACL values (27.9–28.2) in the central part of the fjord may reflect the

24 particularly high precipitation that occurs on the western slope of the Andes immediately to the west of the icefields (Garreaud et al., 2013; Lenaerts et al., 2014). Likewise, the high CPI values indicate that the long-chain n-alkanes are predominantly derived from terrestrial sources, i.e., land-plants, suggesting a low contribution of petrogenic carbon to the fjord sediments, in agreement with the granitoid nature of the regional bedrock (Sernageomin, 2003). The significant correlation between CPI and ACL (r=0.58; p<0.001) results from the fact that both indexes are obtained using n-alkane from higher plants (Jeng, 2006).

5.5. Proxies for terrestrial input

Several of the proxies analyzed here are designed to reflect terrestrial input: the N/C molar

13 ratio, δ Corg, TAR, and the proportion of freshwater diatoms. All of them are independent of accumulation rates. In this section, we compare the capability of these indicators to reflect the terrestrial-to-marine gradient that occurs in the Baker-Martinez fjord system in order to identify the best proxies for terrestrial input. To do so, we represent their variations with distance from the river mouths/calving glacier fronts (Fig. 11). Distance is a good indicator of terrestrial input, as most of the sediment is transported in the upper freshwater lens, making sediment transport independent of variations in bathymetry (Fig. 2).

13 As discussed in the previous section, δ Corg is much more appropriate than N/C to calculate the proportions of terrestrial and aquatic organic carbon, mostly due to the significant influence of lithogenic N on the N/C measurements. When looking at variations across the terrestrial-marine continuum of our study area (Fig. 11), it is evident that the fraction of terrestrial organic carbon

13 calculated from the bulk δ Corg measurements progressively decreases with distance from the river mouth/calving glacier front (Fig. 11a). By comparison, N/C shows very little changes with distance (Fig. 11b), confirming that the lithogenic N contribution renders the proxy inapplicable in

25 most of the fjord system. This is in contrast with the non-glaciated fjords of northern Chilean

13 Patagonia, where N/C and δ Corg co-vary and can generally be used to estimate the proportion of terrestrial organic carbon in sediments (Sepúlveda et al., 2010; Silva et al., 2011; Bertrand et al.,

13 2012b). These differences suggest that δ Corg is much more universally applicable than N/C in fjord environments to reconstruct variations in organic matter sources through space and time.

End-member mixing models for fjord sedimentary organic matter should therefore be based on

13 δ Corg and not N/C. This observation also explains why differences were observed between the

13 fractions of terrestrial organic carbon calculated from δ Corg and N/C measurements in sediment cores from proglacial Chilean fjords (e.g., Bertrand et al., 2012a, 2017), and they suggest that the discrepancies between the two variables may actually be used as an indicator of glacial activity.

Although the TAR values show a slightly decreasing trend with distance from the river mouths/calving glacier front (Fig. 11c), the values remain high (~10) across the fjord system due to land plants containing more n-alkanes than aquatic organic matter. Even at the most oceanic site, which only contains 20% terrestrial organic matter, the TAR value is clearly terrestrial (7.8–15.2;

Fig. 11c). TAR is therefore rather insensitive to changes in terrestrial input, due to the signal being almost entirely controlled by land plants. Its use in fjord sediments is therefore not recommended.

The distribution of freshwater diatoms in the sediments of the Baker-Martínez fjord system reflects input from the continent (Fig. 8d). The major difference with the trend in the fraction of terrestrial carbon (Fig. 11) is that freshwater diatoms quickly drop within the first 40 km of the river mouths/calving glacier fronts, whereas the fraction of terrestrial carbon displays a gradual decrease over the 180 km of the fjord transect. Further offshore than 40 km from river mouths and calving glacier fronts, the proportion of freshwater diatoms remains relatively stable at ~20%, suggesting that diatoms are most sensitive in proximal environments (Fig. 11d). Also, there seems to be no major difference between proglacial and non-glacial fjords, making the freshwater diatom

26 proxy as usable as δ13C to reconstruct changes in terrestrial input, irrespective of the sources of freshwater (river discharge/calving). The main advantage of this proxy is that it might be less dependent on the evolution of terrestrial ecosystems (e.g., vegetation cover) than δ13C, making it readily applicable on longer timescales. Due to its apparent insensitivity to terrestrial inputs at mid- and outer-fjord sites, its use should be restricted to relatively proximal fjord environments.

13 Thus, it appears that δ Corg and the proportion of the freshwater diatoms constitute reliable proxies of terrigenous input in the Baker-Martínez fjord system. They are the best candidates to reconstruct changes in river discharge and precipitation, or meltwater input, from fjord sediment cores. One should however keep in mind that these proxies are relative, i.e., a decrease in marine productivity would appear similar to as increase in terrestrial input.

6. Conclusions

The terrestrial-marine continuum of the Baker-Martinez fjord system allowed us to investigate spatial variations in marine and terrestrial indicators, and to identify the best tracers of terrestrial input. Results show that the signals occurring in the water column and recorded in sediments are predominantly driven by the input of lithogenic particles and terrestrial organic carbon from rivers and calving glaciers. As a consequence, organic carbon, biogenic silica, and carbonates increase with increasing distance from the fjord head. Among all the terrestrial input indicators that were investigated in this study, the fraction of terrestrial organic carbon calculated from bulk δ13C measurements and the proportion of freshwater diatoms are by far the most promising proxies to reconstruct changes in terrestrial input through time using fjord sediment cores. Freshwater diatoms are most sensitive within the first 40 km from the fjord head, whereas the fraction of terrestrial organic carbon is better suited for sediment cores collected in mid and outer fjords.

The main advantage of these two proxies is that they are independent of the glacial status of the

27 fjords and their watersheds, which means that they are applicable to reconstruct terrestrial input in both proglacial and non-glacial fjords. By comparison, n-alkane-based proxies are less promising owing to the consistent terrestrial bias of the signal due to the disproportionally high production of n-alkanes by land plants. Likewise, our results show that the N/C ratio is mostly unsuitable to reconstruct the sources of organic matter due to the relatively high contribution of lithogenic nitrogen, especially in carbon-poor proglacial environments. Based on these results, we recommend focusing on δ13C and/or diatom analysis to reconstruct past changes in terrestrial input from fjord sediment cores. This study therefore sets a baseline for future sediment-based research in the Baker-Martinez fjord system and in Patagonian fjords in general, and it supports the use of fjord sediment archives for paleoclimate and paleoenvironmental reconstructions.

7. Acknowledgments

We thank Dr. Marcelo Gutiérrez (UdeC) for kindly providing the Jorge Montt sediment samples, and Dr. Juan Placencia (UCSC) for sharing some of the CF20 samples used in this study.

Paola Reinoso and Francisco Gallardo (PUCV) are acknowledged for their help in collecting the

CF14 sediment samples for bulk organic geochemical analysis. We also thank Dr. Roberto

Rodríguez (UdeC) for his help with the identification of vascular plants from the Tortel area, and

Víctor Acuña (COPAS Sur-Austral) and Alejandro Ávila (IDEAL Center) for n-alkanes and biogenic opal analyses, respectively. We are grateful to the captain and crew of the L/C Sur-Austral for their help during sample collection, and to the CIMAR program (Cruceros de Investigación Marina en

Áreas Remotas; Marine Research Cruises in Remote Areas). The research presented here was funded by the Center for Oceanographic Research COPAS Sur-Austral (CONICYT PIA PFB31 and

CONICYT PIA APOYO CCTE AFB170006) and by UGent BOF project HYDROPROX (01N02216). Partial support from the IDEAL Center (grant FONDAP 15150003 to CBL and LR), and projects CONA-C14F

28

08-13 and CONA C20F 14-06 (to NS) is also acknowledged. The authors wish to thank Rolf Kilian and an anonymous reviewer for their valuable comments.

29

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Figure captions

Figure 1 – Location of the Baker-Martínez fjord system in Chilean Patagonia, with indication of the main glaciers and rivers influencing the fjord system. Sediment sampling stations are represented by red circles (see Table S2 for details). SPI = Southern Patagonian Icefield.

Figure 2 – Bathymetry of the Baker-Martínez fjord system (from Piret et al., 2017) and location of the CTD stations investigated during the October 2014 COPAS Sur-Austral campaign (top). The cross-sections of temperature, salinity, dissolved oxygen concentration, turbidity, and fluorescence (from top to bottom) are shown for Martínez Channel (left) and Baker Channel

(right). CTD data were plotted using Ocean Data View (Schlitzer, 2017). The bathymetry is not shown on the cross-sections due to its relatively low resolution but shallow (<100m) sills likely occur in the center of Martínez Channel. The color scales are open-ended. Note the change in depth scale for the lowermost (turbidity and fluorescence) cross-sections.

Figure 3 – Principal Component Analysis (PCA) of the four main environmental variables

(temperature, salinity, turbidity, and fluorescence) measured in the upper 20m of the water column at the 17 stations shown in Fig. 2, during four campaigns in spring (October 2014, October

2015) and summer (January 2016, March 2016). For each variable, the averages of all measurements obtained in the upper 20m of the water column were used. The results show high covariance between fluorescence and salinity, suggesting that salinity is the main environmental variable controlling the distribution of phytoplankton biomass in the Baker-Martínez fjord system.

Figure 4 – Spatial and temporal variability in phytoplankton biomass in the Baker-Martínez fjord system. (a) Chlorophyll-a concentrations in October 2014 (austral spring) integrated over the

42 upper 30 m of the water column (COPAS 2014 campaign; data in Table S1). Underlined labels indicate stations where phytoplankton samples were collected in the water column and analyzed

(results in Table S5). (b) Monthly statistics for the position of the 10 and 20 PSU surface salinity contours along Martínez channel, based on hourly records of salinity gathered from five stations

(numbers 2 to 6) along the channel from October 2012 through November 2013. Symbols correspond to the median position of the contours for each month in the record. Error bars correspond to the first and third quartiles (25% and 75% percentiles) of the distribution for each month. Based on the strong covariance between fluorescence and salinity (Fig. 3), a significant increase in phytoplankton biomass is expected to occur between stations 4 and 5 through most of the year, except for summer months (Dec–Feb) when it moves to the outer section between stations 5 and 6.

Figure 5 – Spatial distribution of the four main components of surface sediments in the Baker-

Martínez fjord system. a) Organic matter (wt %), b) biogenic opal (wt %), c) CaCO3 (wt %), and d) lithogenic particles (wt %). Data presented in Table S2.

Figure 6 – Spatial distribution of bulk organic geochemical parameters in surface sediments of the

13 Baker-Martínez fjord system. a) Corg (wt %), b) N/C (molar ratio), c) δ Corg (‰), and d) proportion of terrestrial organic carbon (wt %). Data presented in Table S2.

Figure 7 – Comparison of n-alkanes distribution and concentration between a) living vegetation at

Caleta Tortel, and b) surface sediments of the Baker-Martínez fjord system. Individual plant species are shown in Fig. S1. The error bars indicate the 1σ standard deviation.

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Figure 8 – Spatial distribution of n-alkanes and diatom-based indices in the Baker-Martínez fjord system. a) ACL, b) CPI, c) TAR, and d) freshwater diatoms (%). The main rivers and glaciers are indicated. Data are presented in Table S2.

Figure 9 – Spearman Principal Component Analysis (PCA) of the main variables measured in the surface sediments of the Baker-Martínez fjord system: Corg, biogenic opal, total diatoms, CaCO3,

13 15 lithogenic particles (LP), δ Corg, δ N, N/C ratio, total n-alkanes, carbon preference index (CPI), average chain length (ACL), terrigenous/aquatic ratio (TAR), and proportion of freshwater diatoms.

13 Figure 10 – Molar N/C versus δ Corg of surface sediment samples from the Baker-Martínez fjord system. The end-member values are from Hebbeln et al. (2000)and Bertrand et al. (2012b): terrestrial: N/C = 0.0624 and δ13C = -27.72‰; aquatic: N/C = 0.130 and δ13C = -19.86‰. The N/C and δ13C measurements obtained on river sediment sample RS11-05 confirm the selection of the terrestrial end-member values.

Figure 11 – Variations of the main terrestrial proxies with distance from the nearest river mouth or

13 calving glacier front: (a) Fraction of terrestrial carbon (fT) based on δ Corg (%), (b) molar N/C ratio;

(c) n-alkane-based terrestrial-to-aquatic ratio (TAR) and (d) proportion of freshwater diatoms (%).

All proxies were plotted with the most terrestrial values towards the top of the figure. The open symbols correspond to samples from proglacial fjord Jorge Montt.

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Highlights

 The Baker-Martinez fjord system covers a complete terrestrial-to-marine continuum

 Variability in sediment composition is mostly explained by distance from river mouths

 Ntot/Corg measurements are strongly affected by glacial clay input

 δ13C and freshwater diatoms are the best indicators of terrestrial input org

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Graphical abstract

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