Changes in terrigenous sediment input to the Brazilian equatorial margin as a
response to climate conditions during the middle and late Holocene
a Oceanographic
Institute of University of São Paulo, São Paulo, Brazil
b School of Arts, Sciences and Humanities of University of São Paulo, São Paulo, Brazil
Nancy K. Taniguchia, Ana Silvia de F. Martinsa, Silvia H. M. Sousaa, Cristiano M. Chiessib
[email protected]
Introduction
Results
Modern climatic conditions
Hemipelagic sediments deposited over continental margins are important
archives of past climatic and oceanographic conditions, since they result from
multiple interactions between continents and oceans. The sedimentary
composition can be related to climatic oscillations on various temporal and
spatial scales (Holz et al. 2007).
This study aims to understand the variations of terrigenous sediments
input and their possible relationship with climatic oscillations during the last
8000 years over NE South America.
Study area
(I)
 High energy marine system characterized by the presence of a western
boundary current (NBC), trade winds, very energetic tidal oscillations and
continental runoff from the Pará, Amazonas and Parnaíba rivers;
 The water masses present in study area are: Coastal Water (CW), Tropical
Surface Water (TSW), South Atlantic Central Water (SACW) and Antartic
Intermediary Water (AIW);
 The precipitation regime and ocean circulation are influenced by ITCZ
position;
 East-west antiphasing of South American precipitation on land.
(II)
Figure 1: Location map, showing the study area with position of core GeoB-16204-2 in red (●). (I) Schematic drawing of the main
surface oceanic currents adapted of Schott et al. (2004). Green regions are upwelling zones and blue regions, subduction zones.
Acronyms are: SEC = South Equatorial Current; nSEC, sSEC = South Equatorial Current north and south of the equator; NBUC =
North Brazil Undercurrent ; NBC = North Brazil Current; EUC = Equatorial Undercurrent; SEUC = South Equatorial Undercurrent;
NEUC = North Equatorial Undercurrent,; NECC = North Equatorial Countercurrent; NEC = North Equatorial Current; AD =
Angola dome and GD = Guinea dome. (II) represents long-term mean CMAP precipitation (shaded — scale at right) and 925 hPa wind
vectors (arrows— scale at bottom) for (a) January and (b) July. (III) longterm mean precipitation (shaded — scale at right) and
streamlines at 300 hPa (streamlines) for (c) January and (d) July adapted of Garreaud et al. (2009).
Materials and methods
Material
•
•
•
•
(III)
Core: GeoB-16204-2
Position: 01°59.75 ’S , 42°20.30’ W
Water depth: 1210 m
Recovery: 230 cm
Mid- and late Holocene
(IV)
Final remarks
 Change of environmental conditions at approximatly 5000 years
cal BP;
 Sea-level variation less than 4 m probably is not significant for
variations of terrigenous input due the position of this corer (1200 m
of water column depth and 80 km of distance from de coast);
(V)
This work is linked to the project ”Response to sedimentantion of Amazon
deforestation, land use and climate variability”(AMADEUS), in partnership
with the University of Bremen. The material was collected on board of
MARIA S. MERIAN, MSM20/3 expedition, in February/2012.
 From 5000 to 8000 years cal BP, a higher percent of sandy
sediments and na increase in Fe/Ca, Ti/Ca, Si/Ca values;
Chronology
•
Figure 3: - Painel showing the results of chronology, Fe/Ca, Ti/Ca, Si/Ca, CaCO3 contents and grain size (sortable silt,
sand, silt and clay) of GeoB-16204-2.
14C
AMS datings of monospecific foraminiferal Globigerinoides ruber
(fraction > 0,150 mm) in 06 samples: 6 cm, 50 cm, 106 cm, 150 cm,
200 cm e 230 cm;
• 14C AMS datings converted to calendar ages using the Calib
software, version 7.0, avaiable at
http://calib.qub.ac.uk/calib/calib.html, with Marine 13 Calibration
Dataset and ΔR=0;
• The age-depth relationship was established using the linear
interpolation (Analyseries)
 From 5000 to present years cal BP, an increase in CaCo3 content
suggests na increase in marine influence;
(VI)
 Increase in the mean sortable silt upwards can indicate a decrease in
the bottom current system during the last 8000 years;
(RN)
Figure 2: (IV) Sea-level curves proposed for the northern Rio Grande do Norte coast, NE Brazil.Brazilian coast adapted of Caldas et al.
(2006). (V) speleothem δ18O record record from Rio Grande do Norte (RN) (Cruz et al. 2009); (VI) February insolation at 10°S (Cruz et
al. 2009).
Fe/Ca, Ti/Ca and Si/Ca
• X-ray fluorescence (XRF);
• Avaatech X-Ray Fluorescence core scanner (University of Bremen –
MARUM);
• 2 cm intervals;
• Fe/Ca, Ti/Ca and Si/Ca: terrigenous input
 Sea-level variation less than 4 m on the coast during the last 8000 years;
Carbonate calcium (CaCO3)
the Northern (Southern) Hemisphere;
• CaCO3 contents were obtained by weight difference before and after
sample acidification with HCl 1M.
NE Brazil with dryer conditions in late Holocene than mid-Holocene.
 mid-Holocene was characterised by an increased (decreased) in summer insolation in
Grain Size and Sortable Silt
References
• Grain size distributions of the samples were measured using a
Malvern Mastersizer 2000 and recorded using standard phi (ϕ)
notation ;
• Sortable Silt: mean grain size of the 10 – 63 µm fraction;
• Sortable silt size has been applied as a proxy for past bottom current
speed in deep ocean basins (Bianchi et al., 1999)
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 Decrease in the current velocity be associated with a shift of the
mean position of the Intertropical Convergence Zone (ITCZ).
Currently the system of NBC shows a seasonal variability influenced
by the ITCZ. Furthermore, Haug et al. (2001) also suggest migration
of ITCZ south for the last 5400 years BP;
 Comparing the results of this work with speleothems δ18O data
from Cruz et al. (2009), higher ratios of terrigenous input occured with
wetter conditions in mid-Holocene and lower ratios of terrigenous
input with dryer conditions in late Holocene. This result can be
associated with the east-west antiphasing of South American
precipitation and may possibly be related to the establishment of arid
tropical climate in the Brazilian NE margin. A similar result was
observed by Prado et al. (2013) and Cruz et al. (2009).
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CRUZ, Francisco W. et al. Orbitally driven east–west antiphasing of South American precipitation. Nature Geoscience, v. 2, n. 3, p. 210-214, 2009.
HAUG, Gerald H. et al. Southward migration of the Intertropical Convergence Zone through the Holocene. Science, v. 293, n. 5533, p. 1304-1308, 2001.
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SCHOTT, Friedrich A.; MCCREARY, Julian P.; JOHNSON, Gregory C. Shallow Overturning Circulations of the Tropical‐Subtropical Oceans. Earth's Climate, p. 261-304, 2004.
The first author is funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP (Process Nº: 2013/10676-1).