ORIGINAL

Variations in bottom water activity at the southern Argentinemargin: indications from a seismic analysis of a continentalslope terrace

Jens Gruetzner & Gabriele Uenzelmann-Neben &

Dieter Franke

Received: 30 September 2010 /Accepted: 8 July 2011 /Published online: 9 August 2011# Springer-Verlag 2011

Abstract Continental slope terraces at the southern Argen-tine margin are part of a significant contourite depositionalsystem composed of a variety of drifts, channels, andsediment waves. Here, a refined seismostratigraphic modelfor the sedimentary development of the Valentin FeilbergTerrace located in ~4.1 km water depth is presented.Analyzing multichannel seismic profiles across and alongthis terrace, significant changes in terrace morphology andseismic reflection character are identified and interpreted toreflect variations in deep water hydrography from LateMiocene to recent times, involving variable flow ofAntarctic Bottom Water and Circumpolar Deep Water. Aprominent basin-wide aggradational seismic unit is inter-preted to represent the Mid-Miocene climatic optimum(~17–14 Ma). A major current reorganization can beinferred for the time ~14–12 Ma when the Valentin FeilbergTerrace started growing due to the deposition of sheeted andmounded drifts. After ~12 Ma, bottom water flow remainedvigorous at both margins of the terrace. Another intensifi-cation of bottom flow occurred at ~5–6 Ma when amounded drift, moats, and sediment waves developed onthe terrace. This may have been caused by a general changein deep water mass organization following the closure ofthe Panamanian gateway, and a subsequent strongersouthward flow of North Atlantic Deep Water.

Introduction

Most work in the Argentine Basin up to now has concentratedon the tectonic evolution during and following break-up (e.g.,Tankard et al. 1995; Thomson 1998; Hinz et al. 1999;Macdonald et al. 2003; Franke et al. 2006, 2007). Theexceptionally thick accumulation of sediments present in theArgentine Basin (Ewing et al. 1964) was concluded to havelargely been derived from the continent, and transported totheir present-day position by turbidity currents (Lonardi andEwing 1971). Only in the last decade was it found that theuppermost sedimentary column of the southern Argentineslope and rise is characterized by a complex seismicreflection pattern including evidence of mound facies, giantsedimentary drifts, and numerous unconformities (Hinz et al.1999). Additionally, a significant contourite depositionalsystem (CDS) at the Argentine margin has been identifiedeven more recently by Hernández-Molina et al. (2009). SuchCDSs are generally attributed to along-slope processes (Stowet al. 2002; Rebesco and Camerlenghi 2008); in the presentcase, this indicates that contour-following currents gen-erated by the thermohaline circulation have played animportant role in shaping the seafloor in the ArgentineBasin. Between 44°S and 48°S a set of continental slopeterraces characterizes the CDS, and reveals both erosion-al (e.g., channels) and depositional (e.g., sediment drifts)features of contourites (Gruetzner et al. 2010; Hernández-Molina et al. 2010).

Today, the thermohaline circulation in the ArgentineBasin is characterized by the interaction of northward-flowing Antarctic water masses (Antarctic IntermediateWater, AAIW; Circumpolar Deep Water, CDW; AntarcticBottom Water, AABW) and southward-flowing NorthAtlantic Deep Water (NADW; Arhan et al. 1999, 2002a,2002b; Carter et al. 2008). The transfer of heat and energy

Responsible guest editor: E. Llave

J. Gruetzner (*) :G. Uenzelmann-NebenAlfred-Wegener-Institut für Polar- und Meeresforschung,27568 Bremerhaven, Germanye-mail: Jens.Gruetzner@awi.de

D. FrankeBundesanstalt für Geowissenschaften und Rohstoffe,Hannover, Germany

Geo-Mar Lett (2011) 31:405–417DOI 10.1007/s00367-011-0252-0

via these water masses constitutes an important componentin maintaining the global ocean conveyor belt. Thus, thesedimentary deposits of the Argentine Basin represent anarchive of profound changes in deep ocean circulation.

This study investigates the internal seismic structure of acontinental slope terrace in 4,100 m water depth, the ValentinFeilberg Terrace, thereby refining an earlier Neogene seismicstratigraphy for the site (Hernández-Molina et al. 2009).Terrace evolution is evaluated in terms of variability inSouthern Ocean paleoceanography, climate, and tectonicsduring the last 17 Ma.

Regional setting

Argentine margin morphology

The Argentine Basin extends over approx. 3.4 million km2

and is bounded by the Rio Grande Rise in the north, theFalkland Plateau in the south, the continental margin in thewest, and the Mid Atlantic Ridge in the east (Fig. 1). It hasbeen suggested that the Argentine margin can be subdividedinto at least four segments (I to IV) bounded by the

Falkland, Colorado, Ventana, and Salado transfer fracturezones (Franke et al. 2007). Each of these large tectonicsegments contains thick (>8,000 m) sediments constitutingthe Argentine sedimentary basin located along the slopeand rise (Hinz et al. 1999; Franke et al. 2007; Hernández-Molina et al. 2010).

An extensive and complex CDS is located in thesouthern part (44–48°S) of the Argentine margin betweenthe Malvinas/Falkland and Bahía Blanca transfer fracturezones (Hernández-Molina et al. 2009). The CDS itselfconsists of two major sectors (Fig. 2a): a submarinecanyon and channel sector in the north, shaped mainly bydown-slope processes, and an escarpment and terracesector in the south, dominated by along-slope sedimentdeposition and erosion (Lorenzo and Wessel 1997;Hernández-Molina et al. 2010).

The terraces (from west to east) named Nágera, PeritoMoreno, Piedra Buena, and Valentin Feilberg (Fig. 2b) areseparated by deep channels and moats. Another prominentfeature of the terrace sector is a partially buried giant drift(Fig. 2b) that creates a bathymetric step at the base of theslope (Hinz et al. 1999; Hernández-Molina et al. 2009),possibly formed by the southward branch of a confined

Fig. 1 Regional bathymetricmap of the Argentine Basin,with the general circulation ofdeep water masses: ACC Ant-arctic Circumpolar Current,AABW Antarctic Bottom Water,LCDW Lower CircumpolarDeep Water, NADW NorthAtlantic Deep Water, UCDWUpper Circumpolar Deep Water;barriers (yellow) and passages(orange) of deep circulation: DPDrake Passage, IOR Islas Orca-das Rise, MEB Maurice EwingBank, M/F Malvinas-Falkland,M/F P Malvinas-Falkland Pas-sage, NGP Northeast GeorgiaPassage, SRP Shag Rocks Pas-sage (modified after Piola andRivas 1997, and Arhan et al.1999). Red lines World OceanCirculation Experiment(WOCE) profiles along theArgentine margin (17) andacross the continental slope ter-race sector (11)

406 Geo-Mar Lett (2011) 31:405–417

large loop of AABW from the Eocene–Oligocene boundary(coevally with the Drake Passage opening) until the MiddleMiocene (Hernández-Molina et al. 2010).

Oceanographic setting

The abyssal circulation in the Argentine Basin (Klaus andLedbetter 1988; Reid 1989) is characterized by the complexinterplay of various water masses (Fig. 3a). The present-daydepths of each of these water masses coincide with thelocations of specific sedimentary terraces on the continentalslope (Fig. 3b; Hernández-Molina et al. 2009).

Deep and bottom waters of southern origin enter theArgentine Basin through three main north–south-trendingopenings (Fig. 1): (1) the saddle of the Falkland Plateau (silldepth 2,500 m), located between the Falkland Islands andthe Maurice Ewing Bank (1,200 m), (2) the FalklandPassage between the Falkland Ridge and Islas Orcadas Riseat 35–40°W, with the 5,100-m-deep narrow Falkland Gap at36°W, and (3) the >4,500-m-deep region extending east of

the Islas Orcadas Rise. The net volume of this transport(~100 Sv; Arhan et al. 2002a) is derived from variouscomponents (Gordon 1975; Reid 1989; Arhan et al. 1999,2002b; Orsi et al. 1999; Carter et al. 2008): (1) Weddell SeaDeep Water (WSDW), the deepest water mass in the basin(water depths >4,200 m), (2) South Pacific Deep Water(SPDW, ~3,500–4,200 m), flowing via the Antarctic Cir-cumpolar Current toward the basin after exiting the ScotiaSea through the Shag Rocks Passage, (3) Lower Circumpo-lar Deep Water (LCDW, ~2,000–3,500 m), and (4) UpperCircumpolar Deep Water (UCDW, ~1,000–2,000 m).

LCDW, SPDW, and WSDW contribute to AABW,which flows northward along the Argentine margin as anintensified western boundary current and enters theBrazil Basin through the Vema and Hunter channels(Speer et al. 1992; Hogg et al. 1999). A considerable partof AABW turns eastward at the Rio Grande Rise to forman anti-cyclonic gyre in the Argentine Basin (Fig. 1).NADW enters the Argentine Basin from the north (Fig. 3)at depths of ~2,000 to 3,000 m. The core of NADW flow

Fig. 2 a Bathymetric map of the Argentine Basin (extracted fromSmith and Sandwell 1997) showing the locations of CDS sectors,transfer fracture zones (TFZ), tectonic segments (based on Franke etal. 2007), and the study area (red box). b Shaded relief bathymetry of

the study area, with the locations of continental slope terraces (black),other morphologic features (white) and seismic lines reported in thisstudy (red lines)

Geo-Mar Lett (2011) 31:405–417 407

today occurs seaward of the AABW flow, slightlydetached from the margin (Fig. 1). In the terrace sectorat 45°S (World Ocean Circulation Experiment (WOCE)section 11, Fig. 3b), the presence of diluted NADW isdetected by a local salinity maximum (34.8 psu) at 57°W.

Deep seismic stratigraphy

The Argentine continental slope is characterized byseaward-dipping reflector sequences (SDRs), which devel-oped during seafloor spreading. The upper boundary(reflector AR1) of the SDRs (minimum age Hauterivian,125 Ma) occurs as a distinct erosional surface along theArgentine outer shelf (Hinz et al. 1999), and marks the topof the oceanic crust in the deep sea. The emplacement ofthe SDRs was followed by the deposition of an early-driftsuccession in Hauterivian to Albian times. The top of thissequence, reflector AR2 (Fig. 4), is considered to representthe equivalent of reflector AII from the conjugate CapeBasin, which there has been drilled and found to be of lateAptian age (Ryan et al. 1978). The sedimentation followingthis period is topped by erosional surface AR3 (Hinz et al.1999), named the Pedro Luro equivalent by Franke et al.(2007; Fig. 4). Based on a comparison with well data fromthe shelf (Bushnell et al. 2000), this reflector is interpreted

to mark the Cretaceous/Tertiary boundary. Two seismicreflectors identified by Hinz et al. (1999) are interpreted toresult from the variable deep water current system leadingto the formation of drifts and winnowing during thePaleogene (reflector AR4, Late Eocene), and a renewedcooling and major expansion of the Antarctic ice sheet inMiddle Miocene times (reflector AR5).

Materials and methods

The multichannel seismic (MCS) reflection profiles evalu-ated here (Fig. 2b) were collected by the Bundesantalt fürGeowissenschaften und Rohstoffe during cruise BGR04(Neben and Schreckenberger 2005). These profiles arelocated in tectonic segment I defined by Franke et al.(2007), proximal to the source of deep and intermediatewater masses (AAIW, AABW, CDW) that enter theArgentine Basin from the south.

Seismic signals were generated every 50 m using airgunsin six groups with a total capacity of 3,124 inch3. Data wererecorded for 14,336 ms at a sampling rate of 4 ms using a3-km-long streamer (Neben and Schreckenberger 2005).Processing routines included pre-stack deconvolution,wave-number and frequency filtering and multiple attenu-

Fig. 3 a Salinity (color coded)and neutral density (isolines)section along the Argentinecontinental margin (WOCEline 17, see Fig. 1 for location).AABW Antarctic Bottom Water,AAIW Antarctic IntermediateWater, LCDW Lower Circum-polar Deep Water, NADW NorthAtlantic Deep Water. b Salinitysection across the terrace sectorat the Argentine margin(WOCE line 11, see Fig. 1 forlocation). Terraces: N. Nágera,P.B. Piedra Buena, P.M. PeritoMoreno, V.F. Valentin Feilberg(compiled with Ocean DataView; Schlitzer 2010)

408 Geo-Mar Lett (2011) 31:405–417

ation, velocity analysis, stacking, deconvolution after stack,and FK or Kirchhoff time migration (Franke et al. 2007).Seismic sections are shown in two way traveltime (TWT).Depth estimates and thickness calculation of seismic unitsare based on a velocity–depth model derived fromsonobuoy data by Houtz (1977).

The ages of regional reflectors above AR6 were roughlyestimated for four representative stratigraphic sectionsselected on the MCS profiles BGR2004-06 andBGR2004-08 across the terrace sector. These sections(Table 1) were chosen according to the seismic character-istics of the deposits in areas of continuous, predominantlyhemipelagic deposition, and without major erosional fea-tures. The age estimates were calculated using the meansedimentation rate between AR6 and the seafloor, based onthe calculated reflector depth in meters and the inferred ageof AR6 (~14 Ma).

Results

In the southernmost Argentine Basin, typical margin-parallelmorphosedimentary features are, fromwest to east, (1) a set offour terraces on the slope and rise, (2) channels separating theterraces, (3) a giant buried drift, and (4) sheeted drifts on theabyssal plain that are marked by flat sea-bottom morphology(Fig. 4; Hernández-Molina et al. 2009).

The ~100 km wide plateau of the Valentin FeilbergTerrace (Fig. 4) coincides with today’s LCDW–AABWinterface at depths of ~3.8–4.2 km. Its topography is ratherflat, with slope gradients between 0 and 0.6°. A margin-parallel channel is located at the western boundary of theterrace, formed by the steep (2.9°) eastern flank of theadjacent Piedra Buena Terrace. This channel (channel 1 ofHernández-Molina et al. 2009), with a ~130 m incision at47.5°S, deepens northward to 920 m at 44.5°S. The wide

Av. sed. rate (cm/103 year) Calculated ages for reflectors (Ma)

AR6–seafloor AR6 VF1 VF2 VF3 VF4

Section 1, SP4380, BGR2004-08 8.9 14.0 11.1 8.8 4.8 2.2

Section 2, SP4601, BGR2004-08 9.1 14.0 11.8 9.5 5.9 2.7

Section 3, SP4380, BGR2004-06 9.9 14.0 12.3 10.2 6.7 3.4

Section 4, SP21250, BGR2004-06 10.1 14.0 13.0 9.1 4.2 2.0

Average 9.5 14 12.1 9.4 5.5 2.6

Table 1 Average sedimentationrates and reflector ages forfour representative stratigraphicsections

Fig. 4 Multichannel seismic reflection profiles (lines BGR2004-06and BGR2004-07) across the southern Argentine slope and rise (seeFig. 2 for location), showing the series of terraces at different waterdepths, and numerous channels and sediment drifts. Regional seismicreflectors AR1–AR5 are based on Hinz et al. (1999), and seismic units

on Hernández-Molina et al. (2009). Arrows indicate the extent ofpresent-day water masses: AABW Antarctic Bottom Water, LCDWLower Circumpolar Deep Water, NADW North Atlantic Deep Water,UCDW Upper Circumpolar Deep Water

Geo-Mar Lett (2011) 31:405–417 409

(70 km) eastern flank of the terrace dips seaward (1.2°)down to the crest of a giant drift body at 5,300 m waterdepth (SP 1,500 in BGR2004-07, Fig. 5). Numerousunconformities, channels, and mound facies characterizethe sedimentary sequence following formation of horizonAR4 in the terrace sector (Fig. 5). The discontinuities canbe traced at the scale of the Valentin Feilberg Terrace as awhole, and are commonly accompanied by a change inseismofacies that can be linked to modifications inoceanographic conditions.

The mound facies comprises asymmetric mounded,elongated sediment bodies, which show a steep and a lessinclined flank (e.g., SP 5,150–5,450, BGR2004-08, Fig. 5).These mounds show discontinuities at their bases and, inplaces, internal discontinuities. The internal reflectors aresub-parallel with mostly moderate amplitudes. They showonlap and downlap onto the discontinuities. In general, the

seismic units forming these mounds are lenticular andupward convex. The vertical stacking of the seismicreflectors shows a lateral migration. Furthermore, sheetedsedimentary bodies are plastered to the slope with sub-parallel internal reflectors (e.g., SP 3,500–5,500,BGR2004-08, Fig. 5), and channel-related mounds (e.g.,SP 16,000–17,000, BGR2004-06, Fig. 6a) exhibitingchaotic reflectors. The mounded features exhibit onlyminor lateral variations in seismic reflection characteristics(Fig. 6b). Following published classifications (Faugères etal. 1999; Stow et al. 2002; Shanmugam 2006; Rebescoand Camerlenghi 2008), these features are interpreted asmounded drifts, plastered sheeted drifts, and channel-related drifts all shaped by bottom currents.

Overall, these depositional features and the fact that theyare oriented parallel, rather than oblique, to the slope(Figs. 2b, 6b) indicate that contour-following current

Fig. 5 Detailed seismic stratig-raphy (lines BGR2004-08 andBGR2004-13) across the Valen-tin Feilberg Terrace. Arrowsindicate the extent of present-day water masses: AABW Ant-arctic Bottom Water, LCDWLower Circumpolar Deep Water,NADW North Atlantic DeepWater

410 Geo-Mar Lett (2011) 31:405–417

transport dominates over down-slope (turbiditic) processesat the southern Argentine margin. The depositional stylechanges further northward where across-slope sedimentaryprocesses are indicated by submarine canyons (Hernández-Molina et al. 2009; Krastel et al. 2011; Fig. 2b).

Above reflector AR5 another reflector showing regionalextent and good lateral continuity was recently identified inthe Argentine Basin (Hernández-Molina et al. 2009). Thisstratigraphic key horizon, here named AR6 (Fig. 4), marksthe top of seismic unit IU that shows a weak to transparentacoustic response. Throughout the Argentine Basin, unit IUexhibits an aggradational stacking pattern with littlevariation in thickness. During deposition of the approx.220–270 m thick unit IU at the Piedra Buena and ValentinFeilberg terraces (Fig. 4), the giant drift in the abyssal plainalso underwent a vertical growth stage (Fig. 5).

The present-day Valentin Feilberg Terrace developedduring deposition of the sedimentary sequence boundedby AR6 and the seafloor, unit UU. This unit is further

subdivided here based on variations in reflection character-istics. Four high-amplitude discontinuities (named VF1 to4) separate five subunits named UUa to UUe (from bottomto top) that can be regionally identified across the terrace.The derived ages for reflectors and stratigraphic unitsaveraged over the area are listed in Table 1.

Unit UUa (bounded by AR6 and VF1) is deeply erodedunder the eastern edge of the terrace plateau and theuppermost drift flank (~SP 1–1,000 in profile BGR2004-07,Fig. 6a). Depositional structures exhibiting internal reflectorsof moderate strength onlap AR6 in the west (~SP 3,600–4,800 in profile BGR2004-08, Fig. 5), and form a moundeddrift attached to the western flank of the giant drift in the east(~SP 2,950–3,300 in profile BGR2004-13, Fig. 5).

Units UUb and UUc both show a vertical stackingpattern with sub-parallel reflectors, and can be classified assheeted drifts. These units onlap AR6 at the western terracemargin (~SP 17,000–20,000 in profile BGR2004-06,Fig. 6a), and are separated by the strong unconformity

Fig. 6 Detailed seismic stratig-raphy a across (lines BGR2004-06 and BGR2004-07) and balong (line BGR2004-14) theValentin Feilberg Terrace.Arrows in a indicate the extentof present-day water masses:AABW Antarctic Bottom Water,LCDW Lower CircumpolarDeep Water, NADW NorthAtlantic Deep Water

Geo-Mar Lett (2011) 31:405–417 411

VF2. While unit UUb is rather transparent, unit UUcexhibits moderate- to high-amplitude internal reflectionsthat migrate upslope. In the east, unit UUb exhibits adownlap termination to unit UUa and the giant drift (~SP1,000–1,400 in profile BGR2004-07, Fig. 6a), whereasreflectors in the distal part of unit UUc are truncated alongthe terrace flank (~SP 1,000–1,500 in profile BGR2004-07, Fig. 6a).

Unit UUc is terminated by reflector VF3, which marksthe base of an elongated mounded drift (SP 5,100–5,300in profile BGR2004-08, Fig. 5), the formation of whichbegan at the eastern edge of the Valentin Feilberg Terraceduring deposition of unit UUd. The drift mound has awidth of ~12 km, a height of ~300 m, and exhibits a steepeastern slope and a gentler western slope.

The strong internal reflectors within unit UUd (SP4,500–5,300 in profile BGR2004-08, Fig. 5) indicate aneastward drift crest migration until it became partly erodedduring the formation of subunit UUe (reflector VF 5). In thecenter of the Valentin Feilberg Terrace, the sediments of unitUUd bounded by VF4 and the seafloor fill an erosionaltrough (SP 4,500–4,900 in profile BGR2004-08, Fig. 5)that developed during deposition of UUc.

The uppermost unit UUe also exhibits internal reflectorsprograding seaward into the mounded drift. The drift crestis partly eroded within this unit. Toward the westernmargin, reflections become wavy and finally terminate ina mounded structure with a chaotic internal reflectionpattern that comprises the proximal part of units UUd andUUe at the flank of the Piedra Buena Terrace.

Discussion

Mid-Miocene climatic optimum

In Fig. 7, a stratigraphy for the last ~17 Ma is developedbased on the modification and extension of the regionalseismostratigraphic framework provided by previous inves-tigations (Franke et al. 2007; Hernández-Molina et al.2009). Above horizon AR5 (estimated age 15 Ma, Hinz etal. 1999), the aggradational unit (IU) terminated byreflector AR6 is prominent in the Argentine Basin(Hernández-Molina et al. 2009). In contrast to units LUand UU that exhibit lateral thickness variations due toalternating depositional and erosional features, unit IU

Fig. 7 Summary of depositional sequences above reflector AR4 on the Valentin Feilberg Terrace. * Hinz et al. (1999), ** Hernández-Molina et al. (2009)

412 Geo-Mar Lett (2011) 31:405–417

reveals only minor thickness variations and does not showany indication of erosion (Figs. 4, 5, and 6) over theterraces. It is therefore possible that this aggradational unithas been deposited during the Mid-Miocene climaticoptimum (MMCO), a time of relative sea level highstand(Haq et al. 1987; John et al. 2004), minimum ice volume(Holbourn et al. 2005), and high mid-latitude temperatures(Zachos et al. 2001). The timing of the MMCO is relativelywell constrained from orbitally tuned oxygen isotopestratigraphies to ~17 to 14 Ma (Holbourn et al. 2007). Asimilar aggradational unit tentatively dated between ~18.7 and12.1 Ma was found in the Jane Basin and Weddell Sea (unit 4of Maldonado et al. 2005). Hence, a refined age of ~17 Ma(onset of the MMCO) is suggested here for reflector AR5,while for reflector AR6 an age of ~14 Ma (termination of theMMCO) can be assigned (cf. below). The sedimentthickness above reflector AR5 at SP 4,500 in profileBGR04-08 is calculated to be ~1.5 km. With an inferredage of ~17 Ma for reflector AR5, the average sedimentationrate over the Valentin Feilberg Terrace since the MiddleMiocene would have been >8 cm/1,000 years.

The rather transparent acoustic response of unit IUsuggests steady hydrographic conditions, while the absenceof erosional features could indicate moderate to low bottomcurrent activity. Despite this inferred sluggish circulation,sedimentation rates during deposition of unit IU areestimated to be high (approx. 8–9 cm/1,000 years). How-ever, high continental erosion rates due to the presence of aclimatic (e.g., temperature and precipitation)–continentalsilicate weathering feedback mechanism have been postu-lated for the Middle Miocene (Wan et al. 2009); throughenhanced riverine sediment supply, this would have resultedin higher sedimentation rates in the deep sea. Furthermore,regional subsidence (Van Andel et al. 1977) combined withdown-slope transport processes at the steep eastern flank ofthe Piedra Buena Terrace may have contributed to theapron-like appearance of unit IU.

Subsequently, reflector AR6 can be related to theMiddle Miocene climate transition, a significant reorga-nization of Earth’s climate system characterized bySouthern Ocean cooling and expansion of the Antarcticcryosphere (Flower and Kennett 1994). That cooling tookplace between 14.2 and 13.8 Ma (Shevenell et al. 2004;Holbourn et al. 2007) is evidenced by a step-like increase(Flower and Kennett 1994; Zachos et al. 2001) in the long-term Cenozoic δ18O record, leading to the inception ofcontinuous icehouse conditions in Antarctica (Billups andSchrag 2002; Shevenell et al. 2008).

Valentin Feilberg Terrace evolution

At the Argentine margin a new morphology of the terracesdeveloped after deposition of unit IU, most likely due to

profound changes in bottom flow patterns at ~14 Ma(Hernández-Molina et al. 2009). A stronger flow at the depthof the modern AAIW/UCDW boundary (today at ~2,000 m)is indicated by strong erosion on the Piedra Buena Terrace(Fig. 4). On the other hand, the Valentin Feilberg Terracestarted growing due to the deposition of sheeted andmounded drifts, which at their distal ends partly coveredthe giant drift in the deep sea (Fig. 6a). These changesindicate that the strong return flow of AABWover the terracepostulated to have generated the giant drift (Hernández-Molina et al. 2010; Fig. 8a) became weaker or changedposition after ~14 Ma (Fig. 8b). This could be due to adeepening of the Vema Channel, which before ~14 Ma waspossibly too shallow to enable a deep connection betweenthe Argentine and Brazil basins (Kennett 1982).

Subunit UUa, deposited during the late Middle Miocene(~14–12 Ma), shows sheeted drifts in the west (SP 3,600–4,700, BGR2004-08, Fig. 5) and a small mounded drift inthe east (SP 2,950–3,300, BGR2004-13, Fig. 5). The step-like morphology of the Valentin Feilberg Terrace initiatedwith the deposition of unit LU was leveled again into agently dipping slope (Fig. 8c). A westward-shifted driftonlapping the western flank of the giant drift crest incombination with the small dimensions of the moundsuggest that the AABW flow developed another (weaker)branch westward of the giant drift during the deposition ofunit UUa (Fig. 8c). On the other hand, unit UUa is deeplyeroded under the upper Valentin Feilberg Terrace flank inthe east (SP 5,175–5,585, BGR2004-13, Fig. 5), indicatingvery strong bottom current activity. This could be due to adeepening of the interface between CDW and AABWcaused by the emergence of LCDW during the MiddleMiocene (Nisancioglu et al. 2003; Hernández-Molina et al.2009). Unit UUa is topped by reflector VF1 that istentatively dated to ~12 Ma, and thus coincides withreflector c of Maldonado et al. (2005), the most significanterosional surface within the Jane Basin. A major unconfor-mity of similar age has also been reported for the ScotiaSea, where it is interpreted to represent the first incursion ofAABW in form of WSDW into this basin (Maldonado et al.2006). The event seems to be coeval with a major Mioceneglaciation (Mi4) and a lowering of sea level (Ser3;Maldonado et al. 2006).

Unit UUb topped by high-amplitude reflector VF2 isa mainly aggradational subunit deposited between ~12and ~9 Ma in the confined setting between thecontinental margin and the giant drift (Fig. 8c). Itexhibits vertical growth in its distal part including thecrest of the small drift mound (SP 2,900–3,100,BGR2004-13, Fig. 5), indicating reduced currentstrength. In contrast, truncated internal reflectors (SP17,250–18,500, BGR2004-06, Fig. 6a) indicate sedimenterosion proximal to the Piedra Buena Terrace flank likely

Geo-Mar Lett (2011) 31:405–417 413

due to higher bottom current energy of a more confinedflow of LCDW. This is in agreement with evidence for aMiddle Miocene increase in the deep water circulation inthe Southern Hemisphere presented by Carter et al. (2004)and Joseph et al. (2004).

The confined sheeted drift comprising subunit UUcgrew during the Late Miocene (~9 to ~6 Ma). Its well-stratified homogenous appearance suggests sedimentationunder moderate bottom water flow over wide areas of theterrace (Fig. 8d). Continuous bottom flow is also indicatedin areas displaying wavy reflectors (SP 4,000–4,300,

BGR2004-08, Fig. 5). Locally, high-amplitude internalreflector sequences likely reflect a cyclic sedimentationpattern interpreted as variable current activity over time.Erosional features are extensive at the margins of subunitUUc: in the east, truncated reflectors at the distal terraceboundary indicate that a vigorous bottom current shapedthe terrace flank into its present-day form (Fig. 8d).Focused northward flow along the Piedra Buena Terraceflank continued during deposition of UUc, as suggested byan evolving moat structure that truncates the reflectorsonlapping AR6 (SP 3,100, BGR2004-8, Fig. 5). Taking

Fig. 8 Evolutionary sketches for the development of the ValentinFeilberg Terrace in six stages at various water depths from 17 Ma toRecent. Symbols Inferred bottom water flow: circled cross northward

flow, circled dot southward flow (size indicates current strength). Ddeposition, E erosion, gd giant drift

414 Geo-Mar Lett (2011) 31:405–417

into account paleo-water depths of >4.6 km for the easternterrace flank and ~4.1 km for the western margin (Fig. 8d),erosional features were likely shaped by AABW andLCDW, respectively.

Another profound change in the terrace architecture ismarked by Late Miocene reflector VF3, coeval withreflector b (~6.8 Ma) of Maldonado et al. (2005), animportant unconformity in the northern Weddell Sea.Above reflector VF3, a mounded drift developed duringthe Late Miocene/Early Pliocene (unit UUd, 6–3 Ma) inthe eastern sector of the terrace plateau (SP 5,100–5,400,BGR2004-8, Fig. 5). The drift exhibits a steep east sideand a gently dipping, smooth west side. During depositionof unit UUd, the drift grew vertically and its eastern flanksteepened, likely due to erosion through continuednorthward flow of AABW below ~4,300 m. High-amplitude internal reflections prograding westward in theupper part of UUd may be related to cyclic changes incurrent strength due to intensified glacial/interglacialclimate variability during the Late Pliocene (Zachos et al.2001). Stronger bottom flow (LCDW–AABW) during thePliocene is also indicated by a moat under the centralterrace plateau (Fig. 8e), and sediment waves thatdeveloped toward the west of the moat (SP 3,900–4,200,BGR2004-8, Fig. 5). At the western rim of the terrace,channelized northward flow (at a paleo-depth of ~4.0 km,likely LCDW) continued, forming a channel-related driftwith chaotic internal structure (Fig. 8e). The overallindications for strengthening of bottom flow during theLate Miocene/Early Pliocene can be related to strongerthermohaline overturn induced by a gradual cooling of thewater masses that were sinking in the southern oceans(Billups 2002). Furthermore, stronger production ofNADW commencing around 6 Ma (Wright and Miller1996) and an enhanced flow of NADW into the SouthAtlantic following the restriction of the PanamanianIsthmus at ~5 Ma (Haug and Tiedemann 1998) modifiedthe general deep circulation pattern, although it is unlikelythat NADW directly affected the sedimentation at thedepth of the Valentin Feilberg Terrace.

The youngest high-amplitude reflector VF4 (~2.5 Ma)on the Valentin Feilberg Terrace marks the Late Pliocene/early Pleistocene boundary. A very thin sedimentary coverindicates strong erosion between SP 3,600 and 4,200(BGR2004-8, Fig. 5), while the moat that developed duringdeposition of unit UUd was filled up again, indicating thatthe center of strongest erosion over the terrace plateaumigrated westward during the Pleistocene (Fig. 8f). Thewell-stratified, horizontal, seafloor-parallel, high-amplitudereflectors of the moat fill (SP 4,350–5,000, BGR2004-8,Fig. 5) may be related to ponded turbidites (Faugères et al.1999), and thus possibly indicate down-slope transportprocesses.

Conclusions

Seismic analyses of contourites on a southern Argentinecontinental slope terrace, the Valentin Feilberg Terrace,reflect variations in bottom current flow at 4,100 m waterdepth. Based on changes in depositional style during terracegrowth, a sequence of flow variations for the last 17 Ma isdeduced. Up to now no wells have been drilled in thesouthern Argentine Basin, and thus the dating of the majorunconformities is quite speculative: (1) low bottom currentactivity during the Mid-Miocene climatic optimum (approx.17–14 Ma); (2) current reorganization through the emer-gence of LCDW at approx. 14–12 Ma; (3) moderate flowspeed of LCDW and AABW from approx. 12–6 Ma; (4)strengthening of bottom flow after 6 Ma due to a generalchange in deep water mass organization following theclosure of the Panamanian gateway.

The available multichannel seismic profiles constitute ahigh-quality site survey for drilling sedimentary deposits oncontinental slope terraces in different water depths, whichwould reveal important aspects of deep water history andprovide constraints for the chronostratigraphic framework.The inferred sedimentations rate of >8 cm/1,000 yearswould make a drill site on the Valentin Feilberg Terracesuitable for high-resolution paleoclimate studies of theNeogene.

Acknowledgements The paper benefitted from discussions with F.J.Hernández-Molina. Comments by P. Knutz, E. Llave, two anonymousreviewers, and the journal editors improved the manuscript. This researchwas funded by the Priority Program SAMPLE (South Atlantic MarginProcesses and Links with onshore Evolution) of the Deutsche For-schungsgemeinschaft (DFG) under contract no. Ue 49/11. This is AlfredWegener Institute publication awi-n19434.

References

Arhan M, Heywood KJ, King BA (1999) The deep waters from theSouthern Ocean at the entry to the Argentine Basin. Deep-SeaRes II 46:475–499

Arhan M, Carton X, Piola A, Zenk W (2002a) Deep lenses ofcircumpolar water in the Argentine Basin. J Geophys Res 107:C1. doi:10.1029/2001JC000963

Arhan M, Naveira Garabato AC, Heywood KJ, Stevens DP (2002b)The Antarctic circumpolar current between the Falkland Islandsand South Georgia. J Phys Oceanogr 32:1914–1931

Billups K (2002) Late Miocene through early Pliocene deep watercirculation and climate change viewed from the sub-AntarcticSouth Atlantic. Palaeogeogr Palaeoclimatol Palaeoecol 185:287–307

Billups K, Schrag DP (2002) Paleotemperatures and ice volume of thepast 27 Myr revisited with paired Mg/Ca and 18O/16O measure-ments on benthic foraminifera. Paleoceanography 17:1003.doi:10.1029/2000PA000567

Bushnell DC, Baldi JE, Bettini FH, Franzin H, Kovaks E, Marinelli R,Wartenburg GJ (2000) Petroleum system analysis of the EasternColorado Basin, offshore Northern Argentine. In: Mello MR (ed)

Geo-Mar Lett (2011) 31:405–417 415

Petroleum systems of South Atlantic margins. AAPG Mem73:403–415

Carter L, Carter RM, McCave IN (2004) Evolution of the sedimentarysystem beneath the deep Pacific inflow off eastern New Zealand.Mar Geol 205:9–27

Carter L, McCave IN, Williams MJM (2008) Circulation and watermasses of the Southern Ocean: a review. In: Florindo F, Siegert M(eds) Antarctic climate evolution. Elsevier, Amsterdam, pp 85–114

Ewing M, Ludwig WJ, Ewing JI (1964) Sediment distribution in theoceans: the Argentine Basin. J Geophys Res 69:2003–2032

Faugères J-C, Stow DAV, Imbert P, Viana A (1999) Seismic featuresdiagnostic of contourite drifts. Mar Geol 162:1–38

Flower BP, Kennett JP (1994) The middle Miocene climatic transition:East Antarctic ice sheet development, deep ocean circulation andglobal carbon cycling. Palaeogeogr Palaeoclimatol Palaeoecol108:537–555

Franke D, Neben S, Schreckenberger B, Schulze A, Stiller M,Krawczyk CM (2006) Crustal structure across the ColoradoBasin, offshore Argentina. Geophys J Int 165:850–864

Franke D, Neben S, Ladage S, Schreckenberger B, Hinz K (2007)Margin segmentation and volcano-tectonic architecture along thevolcanic margin off Argentina/Uruguay, South Atlantic. MarGeol 244:46–67

Gordon AL (1975) An Antarctic oceanographic section along 170°E.Deep-Sea Res 22:357–374

Gruetzner J, Uenzelmann-Neben G, Franke D (2010) Seismic imagesof contourites forming continental slope terraces at the ArgentineMargin: implications for past changes in thermohaline circula-tion. Geo-Temas 11:59–60

Haq BU, Hardenbol JAN, Vail PR (1987) Chronology of fluctuatingsea levels since the Triassic. Science 235:1156–1167

Haug GH, Tiedemann R (1998) Effect of the formation of the Isthmusof Panama on Atlantic Ocean thermohaline circulation. Nature393:673–676

Hernández-Molina FJ, Paterlini M, Violante R, Marshall P, de Isasi M,Somoza L, Rebesco M (2009) Contourite depositional system onthe Argentine Slope: an exceptional record of the influence ofAntarctic water masses. Geology 37:507–510. doi:10.1130/g25578a.1

Hernández-Molina FJ, Paterlini M, Somoza L, Violante R, AreccoMA, de Isasi M, Rebesco M, Uenzelmann-Neben G, Neben S,Marshall P (2010) Giant mounded drifts in the Argentinecontinental margin: origins, and global implications for thehistory of thermohaline circulation. Mar Petrol Geol 27:1508–1530

Hinz K, Neben S, Schreckenberger B, Roeser HA, Block M, SouzaKG, Meyer H (1999) The Argentine continental margin north of48°S: sedimentary successions, volcanic activity during breakup.Mar Petrol Geol 16:1–25

Hogg NG, Siedler G, Zenk W (1999) Circulation and variability at thesouthern boundary of the Brazil Basin. J Phys Oceanogr 29:145–157

Holbourn A, Kuhnt W, Schulz M, Erlenkeuser H (2005) Impacts oforbital forcing and atmospheric carbon dioxide on Miocene ice-sheet expansion. Nature 438:483–487

Holbourn A, Kuhnt W, Schulz M, Flores J-A, Andersen N (2007)Orbitally-paced climate evolution during the middle Miocene"Monterey" carbon-isotope excursion. Earth Planet Sci Lett261:534–550

Houtz RE (1977) Sound-velocity characteristics of sediment fromthe eastern South American margin. Geol Soc Am Bull88:720–722

John CM, Karner GD, Mutti M (2004) δ18O and Marion Plateaubackstripping: combining two approaches to constrain latemiddle Miocene eustatic amplitude. Geology 32:829–832

Joseph LH, Rea DK, van der Pluijm BA (2004) Neogene history ofthe Deep Western Boundary Current at Rekohu sediment drift,Southwest Pacific (ODP Site 1124). Mar Geol 205:185–206

Kennett JP (1982) Marine geology. Prentice Hall, Englewood CliffsKlaus A, Ledbetter MT (1988) Deep-sea sedimentary processes in the

Argentine Basin revealed by high-resolution seismic records(3.5 kHz echograms). Deep-Sea Res 35:899–917

Krastel S, Wefer G, Hanebuth TJJ, Antobreh AA, Freudenthal T, PreuB, Schwenk T, Strasser M, Violante R, Winkelmann D (2011)Sediment dynamics and geohazards off Uruguay and the de laPlata River region (northern Argentina and Uruguay). Geo-MarLett (in press). doi:10.1007/s00367-011-0232-4

Lonardi AG, Ewing M (1971) Sediment transport and distribution inthe Argentine Basin. 4. Bathymetry of the continental margin,Argentine Basin and other related provinces. Canyons andsources of sediments. Phys Chem Earth 8:79–121

Lorenzo JM, Wessel P (1997) Flexure across a continent–ocean fracturezone: the northern Falkland/Malvinas Plateau, South Atlantic. Geo-Mar Lett 17(1):110–118. doi:10.1007/s003670050015

Macdonald D, Gomez-Perez I, Franzese J, Spalletti L, Lawver L,Gahagan L, Dalziel I, Thomas C, Trewin N, Hole M, Paton D(2003) Mesozoic break-up of SW Gondwana: implications forregional hydrocarbon potential of the southern South Atlantic.Mar Petrol Geol 20:287–308

Maldonado A, Barnolas A, Bohoyo F, Escutia C, Galindo-Zaldívar J,Hernández-Molina J, Jabaloy A, Lobo FJ, Nelson CH, Rodríguez-Fernández J, Somoza L, Vázquez J-T (2005) Miocene to Recentcontourite drifts development in the northern Weddell Sea (Antarc-tica). Global Planet Change 45:99–129

Maldonado A, Bohoyo F, Galindo-Zaldívar J, Hernández-Molina J,Jabaloy A, Lobo F, Rodríguez-Fernández J, Suriñach E, VázquezJ (2006) Ocean basins near the Scotia–Antarctic plate boundary:influence of tectonics and paleoceanography on the Cenozoicdeposits. Mar Geophys Res 27:83–107

Neben S, Schreckenberger B (2005) Research cruise BGR04:geophysical investigations offshore Argentine and UruguayARGURU. In: Cruise report BGR. Federal Institute for Geo-sciences and Natural Resources, Hannover, p 99

Nisancioglu KH, Raymo ME, Stone PH (2003) Reorganization ofMiocene deep water circulation in response to the shoaling of theCentral American Seaway. Paleoceanography 18:1006.doi:10.1029/2002PA000767

Orsi AH, Johnson GC, Bullister JL (1999) Circulation, mixing, andproduction of Antarctic Bottom Water. Prog Oceanogr 43:55–109

Piola AR, Rivas A (1997) Corrientes en la plataforma continental. In:Boschi EE (ed) Antecedentes históricos de las exploraciones enel mar y las características ambientales. El Mar Argentino y susrecursos pesqueros, Tomo 1. Instituto Nacional de Investigacióny Desarrollo Pesquero, Secretaría de Agricultura, Ganadería,Pesca y Alimentación, Mar del Plata, pp 119–132

Rebesco M, Camerlenghi A (eds) (2008) Contourites. Elsevier,Amsterdam

Reid JL (1989) On the total geostrophic circulation of the SouthAtlantic Ocean: flow patterns, tracers, and transports. ProgOceanogr 23:149–244

Ryan WBF, Bolli HM, Foss GN, Natland JH, Hottman WE, ForesmanJB (1978) Objectives, principal results, operations, and explan-atory notes of Leg 40, South Atlantic. In: Bolli HM, Ryan WBF,McKnight BK, Kagami H, Melguen M, Siesser WG, Longoria JF,Decima FP, Foresman JB, Hottman WE, Natland JH (eds) Initialreports of the deep sea drilling project 40. US GovernmentPrinting Office, Washington, pp 5–28

Schlitzer R (2010) Ocean Data View. http://odv.awi.deShanmugam G (2006) Deep-water processes and facies models.

Elsevier, Amsterdam

416 Geo-Mar Lett (2011) 31:405–417

Shevenell AE, Kennett JP, Lea DW (2004) Middle Miocene SouthernOcean cooling and Antarctic cryosphere expansion. Science305:1766–1770

Shevenell AE, Kennett JP, Lea DW (2008) Middle Miocene ice sheetdynamics, deep-sea temperatures, and carbon cycling: a SouthernOcean perspective. Geochem Geophys Geosyst 9:Q02006.doi:10.1029/2007GC001736

Smith WHF, Sandwell DT (1997) Global sea floor topography fromsatellite altimetry and ship depth soundings. Science 277:1956–1962

Speer K, Zenk W, Siedler G, Pätzold J, Heidland C (1992) Firstresolution of flow through the Hunter Channel in the SouthAtlantic. Earth Planet Sci Lett 113:287–292

Stow DAV, Faugères J-C, Howe JA, Pudsey CJ, Viana AR (2002)Bottom currents, contourites and deep-sea sediment drifts;current state-of-the-art. In: Stow DAV, Pudsey CJ, Howe JA,Faugères J-C, Viana AR (eds) Deep-water contourite systems;modern drifts and ancient series, seismic and sedimentarycharacteristics. Geol Soc Lond Mem 22:7–20

Tankard AJ, Uliana MA, Welsink HJ, Ramos VA, Turic M, FrancaAB, Milani EJ, de Brito Neves BB, Eyles N, Skarmeta J,

Santa Ana H, Wiens F, Cirbian M, Lopez O, Germs GJB, DeWit MJ, Machacha T, Miller RM (1995) Structural andtectonic controls of basin evolution in southwestern Gondwanaduring the Phanerozoic. In: Tankard AJ, Suarez Soruco R,Welsink HJ (eds) Petroleum basins of South America. AAPG,Tulsa, pp 5–52

Thomson K (1998) When did the Falklands rotate? Mar Petrol Geol15:723–736

Van Andel TH, Thiede J, Sclater JG, Hay WW (1977) Depositionalhistory of the South Atlantic during the last 125 million years. JGeol 85:651–698

Wan S, Kürschner WM, Clift PD, Li A, Li T (2009) Extremeweathering/erosion during the Miocene Climatic Optimum:evidence from sediment record in the South China Sea. GeophysRes Lett 36:L19706. doi:10.1029/2009GL040279

Wright JD, Miller KG (1996) Control of North Atlantic Deep Watercirculation by the Greenland-Scotland Ridge. Paleoceanography11:157–170

Zachos J, Pagani M, Sloan L, Thomas E, Billups K (2001) Trends,rhythms, and aberrations in global climate 65 Ma to Present.Science 292:686–693

Geo-Mar Lett (2011) 31:405–417 417