Papers by Barbara Manighetti
Marine Chemistry, 1998
10 laboratories, using their routine methods, collaborated in a comparison of analyses of total c... more 10 laboratories, using their routine methods, collaborated in a comparison of analyses of total carbon, nitrogen and ) Corresponding author. 0304-4203r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Velde, B. (ed.) 1995. Origin and Mineralogy of Clays. Clays and the Environment. xv + 334 pp. Berlin, Heidelberg, New York, Barcelona, Budapest, Hong Kong, London, Milan, Paris, Tokyo: Springer-Verlag. Price DM 138.00, Ös 1007.40, SFr 132.50 (hard covers). ISBN 3 540 58012 3 Geological Magazine, 1997
Marine Geology, 1999
Side-scan, seismic and surficial sediment data accompanied by current meter records highlight acr... more Side-scan, seismic and surficial sediment data accompanied by current meter records highlight across-shelf sediment transport in Hauraki Gulf, an island-studded embayment off northern New Zealand. Calm weather currents are locally dominated by the tides, with periodic incursions of oceanic water from detached meanders of the East Auckland Current. Under these conditions, bedload transport occurs mainly in three 15-20 km-wide channels, where bathymetric intensification of the flow brings about near-bottom speeds of up to 82 cm s y1 for Colville Channel and 33-44 cm s y1 in Jellicoe and Cradock Channels. Surficial sediments are gravelly to muddy sand, winnowed in places, leaving a lag deposit of mainly biogenic carbonate gravel. Modelling results suggest that in Colville Channel, dominant fine to medium sand modes are mobile for 20-60% of the time, with a net eastward movement for fine sand. In Jellicoe and Cradock Channels, the prevailing direction of transport is southwards across the shelf, with sand mobile for up to 33% of the time. Oceanic incursions have the potential to boost flow in the western Gulf, however such incursions are transitory, and there is no measurable expression of oceanic water in the sedimentary record. Because of their association with prolonged periods of calm weather, the incursions are unlikely to accompany storm events, where their cumulative effect might be important for sediment transport. Near-bottom currents resulting from oceanic incursion may reinforce peak tides inside the Gulf by up to 2-4 cm s y1 . Enhancement of prevailing water motions occurs during periods of extreme weather. During cyclone Drena Ž .
New Zealand Journal of Geology and Geophysics, 2003
We identify c. 7000 cal yr BP Tuhua Tephra in a marine sediment core from offshore eastern North ... more We identify c. 7000 cal yr BP Tuhua Tephra in a marine sediment core from offshore eastern North Island, New Zealand. Its mineral assemblage includes aegirine, which is diagnostic for the Tuhua (Mayor Island) Volcanic Centre, and electron microprobe analyses of glass shards yield compositions close to published analyses of the tephra onshore, closer to the source. Associated radiocarbon ages on planktonic foraminifera from the core show that its age lies within the range previously determined for the caldera‐forming event that generated Tuhua Tephra. This occurrence, >380 km southeast of the source, indicates that the dispersal of the ash was bi‐directional, and that earlier estimates of this tephra's volume may be conservative.
Global and Planetary Change, 2002
The last post-glacial transgression and present highstand of sea level were accompanied by a redu... more The last post-glacial transgression and present highstand of sea level were accompanied by a reduction in the terrigenous flux to the deep ocean bordering the active convergent margin off the eastern North Island of New Zealand. Although in accord with long-established models of highstand shelf deposition, new data from giant piston core MD97 2121 (2314 m depth) reveal that the flux also varied with terrigenous supply and palaeocirculation. Between 15 and 9.5 ka, the flux reduced from 33 to 20 g/ cm 2 /ka as supply declined with an expanding vegetation cover, and mud depocentres became established on the continental shelf. An increase from 20 to 27 g/cm 2 /ka during 9.5 -3.5 ka coincided with a strengthened East Cape Current which probably introduced sediment from fluvial and shelf sources in the north. The flux profile shows no immediate response to the establishment of modern sea level f7 ka. However, accumulation decreased from 3.5 to 1 ka as more sediments were retained on the shelf, possibly under wind-strengthened, along-shelf currents. Over the last 1 ka, the flux decline halted under increased terrigenous supply during anthropogenic development of the land.Despite the proximity of the North Island's Central Volcanic Region, major eruptions caused only brief increases (centuries duration) in the terrigenous flux through direct deposition of airfall and possibly fluvial redistribution of onshore volcanic deposits. Frequent earthquakes also had little short-term effect on accumulation although such events, along with volcanism, probably contribute to the long-term high flux of the region.The other measured flux component, biogenic carbonate, reached maxima of 6 g/cm 2 /ka between 11 and 8.5 ka when nutrientbearing waters of the East Cape Current dominated the palaeoceanography. After these peaks, carbonate accumulation declined gradually to modern levels of f3 g/cm 2 /ka. D
Marine Geology, 2006
Sediment input to the deep ocean, off the New Zealand collisional plate boundary, is large and va... more Sediment input to the deep ocean, off the New Zealand collisional plate boundary, is large and variable as revealed by a 139 kyr-old record from the 34.92 m-long core, MD97 2121. Mass accumulation rates (MARs) were derived for terrigenous and biogenic (carbonate and silica) sedimentary components, with temporal control from a stable isotope age model verified by tephra and radiocarbon ages. Terrigenous MARs changed in phase with glacio-eustatic fluctuations of sea level. Highest rates (z 30 g/cm 2 /kyr) coincided with the late regressive-lowstand-early transgressive phase of marine isotope stage (MIS) 2 when rivers discharged at or near the shelf edge. Lesser, but still high terrigenous rates (20-30 g/cm 2 /kyr) characterized peak warm phases of early MIS 5 and 1, when a strengthened Subtropical Inflow probably introduced sediment from fluvial and seabed sources to the north of MD97 2121. However, during prolonged highstands the flux declined as more sediment was retained on a tectonically subsiding shelf with a shoreparallel current. Paradoxically, the lowest MARs (10-5 g/cm 2 /kyr) occurred in MIS 4; either sea level was too high to allow much sediment to escape off-shelf, or the fluvial input was modest, or both these factors.
Palaeogeography Palaeoclimatology Palaeoecology, 2008
The giant piston core, MD97-2121 (2314-m water depth), collected north of the Subtropical Front, ... more The giant piston core, MD97-2121 (2314-m water depth), collected north of the Subtropical Front, New Zealand, provides a well-dated, stable isotopic record of subtropical and sub-Antarctic influences on the surface and deep ocean over the last deglaciation, especially during ...
Paleoceanography, 1995
Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the who... more Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 Ixm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10-1xm grain size, .and noncohesive above that size. Thus silt coarser than 10 Ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed. Silt that is f'mer than 10 Ixm behaves in the same way as clay (< 2 !xm). Useful parameters of the distribution are the 10-63 Ixm mean size and the percentage 10-63 Ixm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, Ku), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface K E of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circmpolar Current, and Brazil/Falldand currents in the Argenthe Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both cocco!iths and foramlnlferal fragments can be recognized and analyzed, with the boundary between them again at about 10 l•m. The flux of less than 10 l•m carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.
Paleoceanography, 1995
Core samples and hydrographic data from 50 ø to 60øN and 15 ø to 25øW in the Northeast Atlantic O... more Core samples and hydrographic data from 50 ø to 60øN and 15 ø to 25øW in the Northeast Atlantic Ocean are used to study changes in current activity from the last glacial to present. We establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis. Changes in current speexl are recorded by the mean size of the sortable (coarse) noncarbonate silt component, which increases with current vigor by winnowing away of fines. Our method involves the definition of an ideal sortable silt "input function", recorded at a site accumulating unmodified pelagic flux only. The ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths. The upper surface of Southern Source Water (SSW) probably shoaled during the glacial, as indicated by the covariation of sortable silt records from sites presently covered by SSW and Lower North Atlantic Deep Water (LNADW). The data suggest that production of LNADW was suppressed during the glacial, increased following the last glacial maximum, and declined at the start of Termination 1A. A second pulse of production occurred immediately before the Younger Dryas event. Intermediate waters were generally faster-flowing during the glacial and may have expanded their depth domain, such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m.
Paleoceanography, 1995
Core samples and hydrographic data from 50 ø to 60øN and 15 ø to 25øW in the Northeast Atlantic O... more Core samples and hydrographic data from 50 ø to 60øN and 15 ø to 25øW in the Northeast Atlantic Ocean are used to study changes in current activity from the last glacial to present. We establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis. Changes in current speexl are recorded by the mean size of the sortable (coarse) noncarbonate silt component, which increases with current vigor by winnowing away of fines. Our method involves the definition of an ideal sortable silt "input function", recorded at a site accumulating unmodified pelagic flux only. The ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths. The upper surface of Southern Source Water (SSW) probably shoaled during the glacial, as indicated by the covariation of sortable silt records from sites presently covered by SSW and Lower North Atlantic Deep Water (LNADW). The data suggest that production of LNADW was suppressed during the glacial, increased following the last glacial maximum, and declined at the start of Termination 1A. A second pulse of production occurred immediately before the Younger Dryas event. Intermediate waters were generally faster-flowing during the glacial and may have expanded their depth domain, such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m.
Paleoceanography, 1995
We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accele... more We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accelerator mass spectrometer dating of a key core, BOFS 5K, and correlation with the rest of the suite. The effects of bioturbation and foraminiferal species abundance gradients upon the age record are modeled using a simple equation. The degree of bioturbation is estimated by comparing modeled profiles with dispersal of the Vedde Ash layer in core 5K, and we find a mixing depth of roughly 8 cm for sand-sized material. Using this value, we estimate that age offsets between unbioturbated sediment and some foraminifera species after mixing may be up to 2500 years, with lesser effect on fine carbonate (<10 µm) ages. The bioturbation model illustrates problems associated with the dating of "instantaneous" events such as ash layers and the "Heinrich" peaks of ice-rafted detritus. Correlations between core 5K and the other cores from the BOFS suite are made on the basis of similarities in the downcore profiles of oxygen and carbon isotopes, magnetic susceptibility, water and carbonate content, and via marker horizons in X radiographs and ash beds.
Paleoceanography, 1995
Fine sediment size (<63 µm) is best measured by a sedimentation technique which records the whole... more Fine sediment size (<63 µm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 µm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10-µm grain size, and noncohesive above that size. Thus silt coarser than 10 µm displays size sorting in response to hydrodynamic processes and its properties may be used to infer current speed. Silt that is finer than 10 µm behaves in the same way as clay (<2 µm). Useful parameters of the distribution are the 10-63 µm mean size and the percentage 10-63 µm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, KM), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface KE of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar Current, and Brazil/Falkland currents in the Argentine Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both coccoliths and foraminiferal fragments can be recognized and analyzed, with the boundary between them again at about 10 µm. The flux of less than 10 µm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.
Paleoceanography, 1995
Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the who... more Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 Ixm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10-1xm grain size, .and noncohesive above that size. Thus silt coarser than 10 Ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed. Silt that is f'mer than 10 Ixm behaves in the same way as clay (< 2 !xm). Useful parameters of the distribution are the 10-63 Ixm mean size and the percentage 10-63 Ixm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, Ku), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface K E of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circmpolar Current, and Brazil/Falldand currents in the Argenthe Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both cocco!iths and foramlnlferal fragments can be recognized and analyzed, with the boundary between them again at about 10 l•m. The flux of less than 10 l•m carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.
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Papers by Barbara Manighetti