Zonal control on Holocene precipitation in northwestern Madagascar based on a stalagmite from Anjohibe

Abstract

The Malagasy Summer Monsoon is an important part of the larger Indian Ocean and tropical monsoon region. As the effects of global warming play out, changes to precipitation in Madagascar will have important ramifications for the Malagasy people. To help understand how precipitation responds to climate changes we present a long-term Holocene speleothem record from Anjohibe, part of the Andranoboka cave system in northwestern Madagascar. To date, it is the most complete Holocene record from this region and sheds light on the nature of millennial and centennial precipitation changes in this region. We find that over the Holocene, precipitation in northwestern Madagascar is actually in phase with the Northern Hemisphere Asian monsoon on multi-millennial scales, but that during some shorter centennial-scale events such as the 8.2 ka event, Anjohibe exhibits an antiphase precipitation signal to the Northern Hemisphere. The ultimate driver of precipitation changes across the Holocene does not appear to be the meridional migration of the monsoon. Instead, zonal sea surface temperature gradients in the Indian Ocean seem to play a primary role in precipitation changes in northwestern Madagascar.

Figure 1

Wind and precipitation for stalagmite collection site, Anjohibe (orange triangle). (a) ERA5 (fifth generation European reanalysis data) at the 850 hectopascal (hPa) atmospheric pressure level with January mean wind direction and strength (red arrows) with Mozambique Channel Trough (MCT) and Mascarene High labeled. (b) ERA5 DJF mean rainfall (blue gradient) with 4 mm isohyet (light yellow line) to denote approximate extent of the tropical rain belt and ERA5 outgoing longwave radiation minima (bright yellow line) to denote approximate position of the summer ITCZ. ERA5 data^– was accessed through the Copernicus Climate Data Store (https://cds.climate.copernicus.eu/) and maps generated with Python version 3 and Cartopy package version 0.20.0.

Figure 2

(a) Age-depth relationship for speleothem AB11. 1000 Monte-Carlo simulations were run in COPRA to generate median ages and confidence intervals. The median age is shown as a solid black line, and the 95% confidence interval is shown with dashed lines. U-Th ages used for the model are shown in purple. Error bars represent the 2σ uncertainties for each age determination. (b) Selected depths sampled for mineralogical analysis using XRD. % Aragonite in orange and % calcite in blue green. (c) Photograph of AB11 approximating depths of samples analyzed in (a) and (b).

Figure 3

Stable carbon and oxygen isotope values of AB11. Top: U/Th ages (purple squares) with 2σ error bars, offset to show overlap. Middle: δ¹³C (‰, VPDB) values of AB11 in orange. Bottom: δ¹⁸O (‰, VPDB) values of AB11 in blue green.

Figure 4

New and existing paleoclimate records from Northwestern Madagascar and Oman. (a) Incoming summer (DJF) solar insolation at 15°S latitude (Anjohibe, dashed line and axis) and summer (June–August, JJA) solar insolation at 30°N latitude (solid line and axis). (b) δ¹⁸O record from Qunf Cave, Oman at 17°N from Fleitmann et al.^,. (c) AB11 δ¹⁸O record from Anjohibe (this study). (d) Anjohibe sample ANJ94-5 δ¹⁸O record from Wang et al. with dry hiatuses (yellow) and wet hiatus (light blue). (e) PCA Axis 2 based on XRF data from Lac Maudit core record from Teixeira et al.. Lime green curve is a smoothed 200 year running average. For (b–e), 30°N insolation curve is also shown on right axes (same as in (a)) and red (dry) and blue (wet) dashed vertical lines denote events referenced in the main text.

Figure 5

(a) World Ocean Atlas 2018 (WOA18) SSTs for decades between 1955–2017^,. Data visualization via Ocean Data View version 4, downloaded from https://odv.awi.de. Areas used to calculate DMI outlined in dashed lines. Sites of ocean gravity and piston cores shown as circles with colors matching Mg/Ca SST records in (b)^,. (c) AB11 δ¹⁸O record from Anjohibe (this study, blue green) and W to E SST gradient (black) for the Indian Ocean based on records in (b). (d) Speleothem δ¹⁸O records from Western Australia (KNI-51) and Flores, Indonesia (Liang Luar). Light purple is record from Griffiths et al. and dark purple is 50-year sea-water corrected composite record from Scroxton et al..