Slowdown of Circumpolar Deepwater flow during the Late Neogene: Evidence from a mudwave field at the Argentine continental slope

Abstract

Geochemical evidence from boreholes suggests enhanced transport of Northern Component Water (NCW) to southern latitudes from about 6 Ma onward. However, information on how this change in transport influenced the intensity and position of current systems is sparse. Here we use seismic reflection profiles interpreted together with bathymetric data to investigate current derived deposits at the central Argentine Margin. Upslope migrating mudwaves overlying a late Miocene erosional unconformity provide evidence that Circumpolar Deepwater flow slowed down with the onset of NCW inflow. During the last 3 Ma, changes in dimensions and migration rates of the waves are small indicating continuous bottom current flow conditions similar to today with only minor variations in flow speed, suggesting that the Deep Western Boundary Current in the western south Atlantic as observed today has been a pervasive feature of the global thermohaline circulation system during the Plio-/Pleistocene.

Evolution of the northern Argentine margin during the Cenozoic controlled by bottom current dynamics and gravitational processes

“…Also the upslope wave migration at the Argentine slope (Figure ) is in agreement with a north setting bottom current flow during the Plio‐Pleistocene since modeling studies [ Blumsack and Weatherly , ] as well as empirical investigations [ Flood et al ., ; Gruetzner et al ., ] suggest that in the Southern Hemisphere, sediment wave migration is commonly to the left of the flow direction. The observed sediment wavefield (40°S–41°S) located north of the mounded sediment drift (41°S–42°S, Figure d) occurs at the same depth level (4000–5000 m) as another wavefield and another mounded drift mapped at 42.5°S–44°S and 44°S–45°S, respectively [ Gruetzner et al ., ]. Figure shows that the waves formed directly above the erosional unconformity AR7 which indicates that deep current velocities at ∼6 Ma slowed down into a range where wave (and drift) growth was possible at the rise.…”

“…The wavy appearance of the internal reflections of this unit indicates bottom flow over long periods of time that could have been caused either by contour currents or by turbidity flow. Although other buried sediment wavefields at the central margin have been attributed to a contouritic origin [Gruetzner et al, 2014], we argue that the wavy structures at the lower slope may predominantly reflect downslope transport processes for the following reasons: (1) indications for currentinduced features of this age interval in other margin segments are very sparse [Gruetzner et al, 2012]. (2) The wavy structures often have an irregular shape, show no clear migration direction or even migrate downslope ( Figure 6, SP 2800-3000).…”

“…Here gravity controlled downslope transport to the deep sea is indicated by numerous submarine canyons [ Lastras et al ., ] and mass transport‐derived deposits of Miocene age [ Gruetzner et al ., ]. However, despite the dominance of the canyons, other morphologic features like terraces, slope plastered drifts and sediment waves [ Gruetzner et al ., ] indicate the influence of bottom water masses, that interact with the downslope processes.…”

“…In this study, we investigate current controlled sedimentation and mass wasting processes at the northern Argentine margin between 37°S and 42°S and thus extend our prior investigations from the southern [ Gruetzner et al ., ] and central [ Gruetzner et al ., ] margin toward the North. The working area (Figure ) is part of the rifted volcanic margin offshore Argentina [ Becker et al ., ], stretches from the outermost continental shelf toward the abyssal plain, and includes the seaward prolongation of the Colorado Basin.…”

A New Seismic Stratigraphy in the Indian‐Atlantic Ocean Gateway Resembles Major Paleo‐Oceanographic Changes of the Last 7 Ma

“…Downslope sediment flows and river‐fed turbidite systems have been described for the eastern margin of southern Africa (Castelino et al, ; Wiles et al, ) but the Site U1475 sediment cores from the elevated top of the AP do not show any turbiditic sedimentary structures (Hall et al, ). Thus, we interpret the sediment waves on the AP, which developed contemporaneously to a wavefield at the same latitude in the western Atlantic (Gruetzner et al, ), to be shaped by contouritic bottom currents. A model of sediment wave formation (Flood, ) predicts that the observed wave dimensions at the AP can form under geostrophic flow velocities that range from ~8 to 17 cm/s and the observation that the wave crests do not exhibit a significant up‐current migration would point toward flow velocities at the lower end of this range.…”