Oxygenation variability o ff Northern Chile during the last two centuries

Introduction Conclusions References


Introduction
Eastern boundary upwelling systems such as Peru-Chile, California, Canary and Benguela are amongst the richest areas for world fisheries concentrating ∼10-20% of world catch in ∼0.1% of the ocean (Chavez et al., 2008;Fre ón et al., 2009).With the exception of the Canary Current these eastern boundary upwelling systems are characterized by the formation of an intense mid-water depth oxygen minimum zone (OMZ) fueled by high levels of organic matter respiration, sluggish circulation and/or the advection of low oxygen waters (Helly and Levin, 2004).The study of such oxygen deficient layers is of particular interest because of their contribution to gas exchanges between the ocean and the atmosphere, including CO 2 , at both regional and global scales (Fre ón et al., 2009).Several studies recently pointed out that during the last Figures

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Full century as climate warmed, anoxia increased in the world ocean (Chan et al., 2008;Diaz and Rosenberg, 2008;Monteiro et al., 2008;Stramma et al., 2008;Keeling et al., 2010 and references therein).In this work we particularly focus on the Peru-Chile Current System's OMZ variability and its relation with biological productivity during the last two centuries.
The relatively shallow (<100-∼600 m deep) OMZ off Peru and Chile is strongly affected at interannual timescales by the occurrence of El Ni ño events during which warm water masses from the Western Equatorial Pacific Ocean intercept with the coastal upwelling zones, oxygenating the shelf and upper slope (Blanco et al., 2002;Arntz et al., 2006;Fuenzalida et al., 2009, and references therein).At longer time scales, enhanced organic carbon burial and higher δ 15 N have been reported from sediments collected in Northern Chile and Southern Peru between AD 1820 and 1878 (Vargas et al., 2007).This shift has been associated with centennial scale intensification of southerly winds, enhanced upwelling of nutrients to the euphotic zone, and increased primary production as well as with less oxygenation promoting denitrification (Vargas et al., 2004(Vargas et al., , 2007;;Siffedine et al., 2008).In this paper we use information provided by redox-sensitive elements Mo, V and S and the (lycopane+n-C 35 )/n-C 31 ratio preserved in the sediments as bottom water oxygenation proxies.The signal of paleoxygenation is examined in a multi-proxy framework taking into account biological productivity and the occurrence of historical El Ni ño Southern Oscillation (ENSO) events.On this basis, we will not only be able to reconstruct the upper sediment and water column redox conditions during the last two centuries, but also to assess underlying mechanisms responsible for oxygenation variations at decadal scales and provide insight on the response of the Peru-Chile upwelling ecosystem to recent climate change.

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Full  Marin et al., 1993), restricted water circulation, low turbulence, and low concentrations of dissolved oxygen in bottom waters (Escribano, 1998;Marin et al., 2003;Vald és et al., 2003;Vargas et al., 2004).The high primary productivity of the Mejillones area is sustained by the quasi-permanent upwelling of nutrient-rich Equatorial Subsurface Water off Angamos Point and the formation of filaments (Marin et al., 2001).However, several studies conducted about the 1997-1998 El Ni ño phenomenon suggested that this general pattern may be altered by the presence of Kelvin poleward waves which increase sea surface temperature and deepen both the thermocline and the oxycline (e.g., Gonz ález et al., 1998;Iriarte et al., 2000;Fuenzalida et al., 2009).
Presently, sediments in Mejillones Bay are deposited under oxygen-depleted conditions and good preservation of organic matter and biogenic proxies such as diatom valves, tests of foraminifers and fish scales is evidenced by the presence of laminated sediments (e.g., Ortlieb et al., 2000;Vald és et al., 2004;Vargas et al., 2004;Díaz-Ochoa et al., 2008;Vargas et al., 2007;Caniup án et al., 2009).High rates of primary productivity promote high export rates of agglomerated (brown and black) organic matter as well as high rates of sulfate-reduction which favor precipitation of metallic sulfurs from bottom water to sediments (Vald és et al., 2004), as observed in other environments (Zheng et al., 2000).On the other hand, periods of lower primary productivity, organic matter deposition, and sulfate reduction rates are recorded as dispersed and Figures

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Full lones Bay.The first core was collected with a box corer at 23 • 03.3 S, 70 • 27.4 W (BC-3; Fig. 1).This core was subsampled on board using four acrylic plates (50×10 cm) and then stored at 2 • C for further analysis in the laboratory.One of the acrylic plates (BC-3C) was subsampled with a resolution of 0.5 cm for dating based on 210 Pb activities (Caniup án et al., 2009), whereas another plate (BC-3D) was used for measuring the contents of biogenic opal (SiO 2 ), organic carbon (C org ), and chlorins, as well as for analyzing physical properties (e.g.water content, dry bulk density; data available from Caniup án et al., 2009).Although data for BC-3D were acquired with laminae resolution, here we present the results averaged in 0.5 cm intervals.Plate BC-3D was dated by lateral correlation with plate BC-3C using magnetic susceptibility profiles (Caniup án et al., 2009).
The second core was collected with a multicorer at 23 • 2.6 S, 70 • 27.1 W (MUC-1B; Fig. 1).MUC-1B was sectioned onboard every 0.5 cm and samples were stored in plastic bags at 4  and Oldfield, 1978;Binford, 1990).At core depths where the CRS could not be applied, we used stratigraphic correlations with other dated cores collected in the same location (i.e.CaCO 3 content of core F981A collected in 1998, Vargas et al., 2007) or in a nearby location (i.e.several stratigraphic features observed in core BC-3 by Caniup án et al., 2009) to extrapolate ages downcore by simple linear regression.In fact we correlated core MUC-1B with core BC-3D using the presence of a "spongy" layer (see description in Sect.4.2) and a disturbed layer caused by dredging works of the Mejillones harbor (Grillet et al., 2001;Caniup án et al., 2009).Conducted at the end of 2002, the works of dredging of the Mejillones harbor produced resuspension of sediment in the upper ∼5 cm of BC-3 associated with higher magnetic susceptibility values and possibly high Fe contents (Caniup án et al., 2009).In core MUC-1B sediment resuspension corresponds to strong fluctuations of density and increased accumulation of Al and Fe.Therefore, we infer that the upper 4.5 cm of core MUC-1B correspond to harbor dredging perturbations and hereon assign to it an age of ∼2003.In addition, an Figures

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Full extremely dense layer near the base of MUC-1B was correlated with a maximal density layer probably associated with an earthquake occurred near Cobija, Northern Chile, in AD 1836 (Caniup án et al., 2009).The chronology of ENSO events occurred during the 19th century was taken from Garcia-Herrera et al. ( 2008) and Gergis and Fowler (2009), and the consistency of the Gergis and Fowler (2009) chronology for the ENSO events until the end of the 20th century was checked with the chronology provided by Trenberth (1997).To facilitate the comparison between ENSO chronologies and our sediment records the total number of El Ni ño and La Ni ña were grouped within the same time intervals established with the age model estimated for core MUC-1B.
For core MUC-1B, we used biogenic opal measured with the method of Mortlock and Froelich (1989) and counts of diatom valves as productivity proxies (S ánchez, 2009) as well as the content of biogenic Ba (bioBa) extracted with 2M NH 4 Cl (pH=7) following the method of Rutten and De Lange (2002) for several intervals in the core.
We also computed biogenic Ba using the normative method (Ba bio ) including total Ba and detrital Ba/Al ((Ba/Al) det ) estimated as the difference between total Ba (Ba tot ) and bioBa and using the equation Ba bio =Ba tot −(Al×(Ba/Al) det ) (Reitz et al., 2004).With a resolution of 0.5 cm, Ba bio calculation took into account the interval between minimal and maximal (Ba/Al) det observed in MUC-1B and provided a reference framework for interpreting lower resolution (∼1 cm) direct measurements of bioBa.Most elements measured by ICP-OES were normalized with Al to correct for variable carbonate content.Percent deviations from the mean were computed for Ti/Al and Zr/Al and their summation was used as a proxy for terrigenous material input.Mo/Al and V/Al were interpreted as paleoredox proxies whereas S, corrected for seawater salt content, was used as a proxy of sulfate reduction intensity (Nameroff et al., 2004 Full In addition, we studied the depth variation of the ratio (lycopane+n-C 35 )/n-C 31 as an organic paleoredox proxy since preservation of lycopane is enhanced under anoxic conditions (e.g., Sinninghe Damst é et al., 2003).Lycopane was extracted from ∼0.6 g sediment samples of MUC-1B and analyzed using an Agilent Technologies 6890 instrument equipped with an Agilent Technologies 5973 mass spectrometer.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.Helium was used as the carrier gas (101.9kPa pressure) with a flow of 24.4 mL/min, the oven was programmed from 80 • C to 130 • C at a rate of 20 • C/min, and then to 310 • C (32.5 min) at a rate of 4 • C/min.The mass spectrometer was programmed to 70 eV and compounds were identified based on the relative retention time and by comparison with mass spectra reported in literature (Kimble et al., 1974;Volkman, 2005).

Chronology
A comparison of several proxy data in cores MUC-1B, F981-A and BC-3 is presented in Fig. 2. In core MUC-1B high and relatively uniform values of 210 Pb, Fe/Al and %CaCO 3 are apparent within the top 4.5 cm whereas Fe/Al and %CaCO 3 display local minima within the spongy layer (see Sect. 4.2) between 20 and 21.5 cm core depth (Fig. 2a,b).The layer affected by the Mejillones harbor dredging works during late 2002 as inferred by increased magnetic susceptibility and rather uniform 210 Pb activities in core BC-3 is also shown (Fig. 2d).In addition, we present the correlation between several %CaCO 3 peaks in cores MUC-1B and F98-1A (Fig. 2b,c showed anomalous 210 Pb distributions that violated the steady state assumption) and stratigraphic correlations with cores F98-1A and BC-3 extrapolated deeper downcore (Fig. 2f,g).
A series of ENSO events based on the recent chronology proposed by Gergis and Fowler (2009) and the list of events since 1950 by Trenberth (1997) is presented in Table 2.In these data are remarkable several periods with maximal frequency of El Ni ño (i.e.four events in 1975-1980, 1937-1941, 1902-1906, and 1845-1850) and La Ni ña events (i.e.four or more events in 1906-1911, 1893-1898, and 1871-1876).

Paleoproductivity proxies
The biogenic opal record displayed a decreasing long-term trend with high values (%SiO 2 ∼>50% w/w in core BC-3D) during the early 1800s and lower and more variable values (%SiO 2 ∼<35% in both cores MUC-1B and BC-3D) throughout the rest of the 19th century.However, since the mid 20th century biogenic opal in both cores recovered and frequently attained values >40% (Fig. 3a).In contrast with long-term trends counter-act with the exception of a brief interval during the early 1870s and during the 1950s, the latter coincident with anomalies of 210 Pb (Fig. 3a,d).Within the spongy layer (∼the 1860s) biogenic opal was quite variable and by the early 1870s the previous decreasing trend disappeared (Fig. 3a).In addition, since 1863, and throughout the rest of the 1860s, diatom valves declined abruptly from ∼10×10 8 g −1 to ∼<2×10 8 g −1 while the contribution of valves of warm-water species (especially those of Rhizosolenia formosa Peragallo) increased by a factor of 3 (Table 1).Coincident, there is a strong decline in C org content (BC-3D).This minimum may also be reflected in Ba bio (MUC-1B) and chlorins (BC-3D) but is not observed in biogenic opal (Fig. 3).Biogenic Ba directly determined from selective extractions (bioBa) oscillated at 210±77 ppm in core MUC-1B (Fig. 3c) whereas more detailed Ba bio , determined from total Ba and the detrital Ba/Al (see Material and Methods section), permitted to describe a general trend for biogenic Ba (the thin lines of Fig. 3c).Moreover, bioBa showed some coherency with biogenic opal content (r=0.57,p<0.01) and diatom valves count (r=0.67,p<0.01) in core MUC-1B (Fig. 3c), but not with S (r=−0.30,p>0.1) nor Mo/Al (r=0.15,p>0.4).Furthermore, for core BC-3D it is remarkable that the productivity proxy chlorins does not correlate with biogenic opal (r=−0.0002,p<0.998) whereas C org is negatively correlated with biogenic opal (r=−0.61,p<0.0001) (Fig. 3).Accordingly, the general depth distribution of biogenic Ba is consistent with that of chlorins and C org , but all of these do not show a clear relationship with biogenic opal and diatom valve counts.

Paleoxygenation proxies
Depth distribution patterns for redox-sensitive element ratios Mo/Al and V/Al are similar and display high and coherent variability (r=0.83,p<0.001;Fig. 4).These patterns are roughly replicated by the (lycopane+n-C 35 )/n-C 31 ratio during several periods (Fig. 4a,b).All the redox-sensitive proxies display a general trend towards higher values to the present, a trend which appears more pronounced for S and Fe/Al proxies which in addition are strongly correlated with the summation of percent deviation from Introduction

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Discussion
Two sediment cores have been collected in Mejillones Bay in Northern Chile, the only setting of the Chilean margin where laminated sediments have been reported (e.g., Ortlieb et al., 2000;Vargas et al., 2004).Using bottom-water oxygenation proxies Mo/Al, V/Al and S, and the ratio (lycopane+n-C 35 )/n-C 31 we have investigated changes in oxygenation conditions between ∼1836 and ∼2002.
Cores MUC-1B and BC-3 were successfully correlated and a constrained age model could be obtained for core MUC-1B in order to study the signals of oxygenation and productivity observed during the last two centuries in Mejillones bay.However, because coastal upwelling areas such as the Peru-Chile Current are simultaneously characterized by high biological productivity and the formation of an Oxygen Minimum Zone, proxies' interpretation is not always straight forward.Consequently, we focus on the interpretation of paleoproductivity proxies as a function of bottom water oxygenation and their related preservation.

Oxygenation and productivity proxies
Redox-sensitive trace metals in core MUC-1B are much enriched, especially since the mid 1850s, suggesting more oxygenated conditions in the bottom water before that time.These observations are consistent with previous findings pointing to a transition period between 1820 and 1878 characterized by stronger upwelling favorable coastal winds off Northern Chile and Peru (Vargas et al., 2007;Fig. 4).
In addition, the spongy layer corresponding to the 1860s seems coincident with a time interval when very strong ENSO events occurred in the Pacific basin (i.e.La Ni ña years: 1860, 1861, and 1863: Gergis and Fowler, 2009;El Ni ño years: 1866 and1868;Garcia-Herrera et al., 2008).As a whole, the spongy layer appears mostly Figures

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Full dominated by La Ni ña like conditions during the early 1860s and concomitantly higher biological productivity (i.e. higher biogenic Ba, chlorins and C org contents) and stronger low oxygen conditions (i.e. higher Mo/Al, V/Al and (lycopane+n-C 35 )/n-C 31 ) in the bottom water.In contrast, towards the late 1860s a simultaneous drop in biogenic Ba, chlorins and C org and lower Mo/Al, V/Al and (lycopane+n-C 35 )/n-C 31 ) as well as minimal S were observed (Figs. 3a and 4a,b).Moreover, the upper boundary of the spongy layer that includes the interval between 1867 and 1869, reported as a period with extended El Ni ño events (Garcia-Herrera et al., 2008), is characterized by a high abundance of the warm water diatom species R. formosa in the sediments (Table 1).Oxygenation proxies Mo/Al and V/Al in core MUC-1B approximately describe a series of peaks and troughs which, to some extent, could be related with ENSO events documented for periods such as the early 1900s or between the 1930s and the early 1940s characterized by a notable reduction in the number of La Ni ña events (Gergis and Fowler, 2009; Table 2).Moreover, Mo/Al and V/Al variability is correlated positively with biogenic opal fluctuations in core MUC-1B.However, as shown for the spongy layer, the relationship between oxygenation/productivity and ENSO activity is obscured by an averaging effect produced by the inclusion of several events within the same sampling interval.Thus, the only way to separate unequivocally the signals associated with particular events would be an analysis at laminae resolution.On the other hand, it seems highly probable that the cycles described by the redox proxies Mo/Al, V/Al and (lycopane+n-C 35 )/n-C 31 would be associated with ENSO-like variability accompanied by biological productivity and oxygenation fluctuations in Mejillones bay.Productivity proxies biogenic Ba, chlorins and C org display a coherent oscillating pattern (Fig. 3c-e) that roughly resembles to decadal cycles such as those proposed by Vargas et al. (2007).According to our data, such pattern is characterized by periods of sustained increase of primary production (e.g. the ∼80 years period of growing productivity between the late 1830s and the early 1920s or the period since the early 1950s) followed by periods with rapid declines like those observed between the 1920s and the late 1940s (Fig. 3c-e).Taking into account that Mejillones bay is a highly pro-

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Full ductive coastal upwelling ecosystem where diatoms are the dominant phytoplankton, it is surprising that biogenic opal signals in cores MUC-1B and BC-3 seem to counteract (Fig. 3a).We speculate that in the study area there might be shifts in diatom blooms depending on the distance from the coast and/or depth or, alternatively, that this counter-acting pattern could be the result of wave action at different depths.

Preservation issues
The close relationship between sulfur content and the Fe/Al ratio (Fig. 4c) strongly suggests that most of the reduced sulfur deposited in the sediment is incorporated into metallic sulfides such as pyrite.It is also supported by the general pattern of biogenic Ba that corresponds to those for chlorins and C org and is not anti-correlated with that of S. Therefore, we suggest that the sedimentary environment in Mejillones bay has become more sulfidic since the mid 19th century and that by the early 1960s it experienced a rapid shift to even more sulfidic conditions (Fig. 4c).This is not only expressed as a long term increase of sulfidic conditions, but also, as a decadal oscillation of enhanced primary and export production that is probably associated to ENSO-like frequency and increased intensity of upwelling favorable winds off Northern Chile (Vargas et al., 2007).In addition, it has been suggested that organic carbon preservation depends on the redox conditions in surface sediments and successive oxygenation oscillations would have the net effect of reducing preservation of chlorophyll and degradation products (Sun et al., 2002).In this sense, the increased frequency of ENSO events documented since the second half of the 20th century (e.g., Trenberth, 1997) might have produced a combined productivity-preservation signal in the sedimentary record of the Mejillones bay.It is interesting to note that only S and chlorins have a generally increasing trend towards the present (i.e.since the 1960s) whereas such trend is not observed for C org suggesting increased preservation of chlorins under stronger sulfidic conditions supported by the evidence of less efficient degradation of chlorophyll in anoxic than oxic sediments (Sun et al., 1993).Based on the latter interpretation, the productivity and Introduction

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Full    Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2005).
) as well as the correlation established by the presence of a spongy layer in cores MUC-1B and BC-3 (Fig. 2a,b,d,e).The final integrated age model for core MUC-1B, resulted from the combination of the CRS model fitted to 210 Pb excess in the upper 10 cm (the interval between 10 and 15 cm Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | at shorter time scales biogenic opal records in MUC-1B and BC-3D seem mostly to Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |bottom-water oxygenation proxies analyzed in this work provide evidence to confirm the hypothesis proposed byVargas et al. (2007)  that the marine coastal ecosystem off Northern Chile has experienced a shift since the mid 19th century towards increased biological productivity due to stronger upwelling favorable winds (i.e.increased input of Ti and Zr, Fig.4d).Moreover, the record of proxies presented indicates that the Mejillones bay have experienced a new and abrupt shift since the early 1960s that is pushing the ecosystem to even more productive and more sulfidic conditions than those produced since the shift occurred between 1820 and 1878.6 ConclusionsThe sediment of Mejillones bay have recorded during the last two centuries what seems to correspond to a shift in the coastal marine ecosystem off Northern Chile characterized by intense ENSO-like activity and large fluctuations of both biological export productivity and bottom water oxygenation.In addition, in the long term, such variability has resulted in a constant increase of sulfidic conditions in the environment, pointing to more sustained oxygen-depleted or even sulfidic (sedimentary) environmental conditions.The latter conditions appear to have intensified since the early 1960s, which may provide insight on the response of the Peru-Chile upwelling ecosystem to recent climate changeDiscussion Paper | Discussion Paper | Discussion Paper | and comparative approaches, Prog.Oceanogr., 83, 1-14, 2009.Fuenzalida, R., Schneider, W., Garc és-Vargas, J., Bravo, L., and Lange, C.: Vertical and horizontal extension of the oxygen minimum zone in the Eastern South Pacific Ocean, Deep-Sea Res.Pt.II, 56, 992-1003, 2009.Garcia-Herrera, R., Diaz, H. F., Garcia, R. R., Prieto, M. R., Barriopedro, D., Moyano, R., and Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 Fig. 1 .Figure 3 Fig. 3 .
Figure 1 • C. A chronology for MUC-1B was established by measuring 210 Pb activities and computing ages with the constant rate of supply model (CRS; Appleby

Table 1 .
Diatom counts in selected core depths around the spongy layer for core MUC-1B collected in Mejillones Bay (Northern Chile).Species are grouped depending of the habitat and/or distribution and expressed as relative abundance (%)(S ánchez, 2009).

Table 2 .
Number of El Ni ño (EN)and La Ni ña (LN) events (years) included in the sampling intervals of core MUC-1B (events before 1950 from Gergis and Fowler, 2009 and after that year from Trenberth, 1997).