DOI: 10.25921/ffxs-yz87
title: NOAA/WDS Paleoclimatology - Central and Northeast India 1000 Year Stalagmite Oxygen Isotope Data

description: The "internally" generated intraseasonal variability of the Indian Summer Monsoon is characterized by intermittent periods of enhanced ("active") and deficient ("break") precipitation, which produce a quasi east-west precipitation dipole over the Indian subcontinent. Here we present multicentennial-length and near annually-resolved reconstructions of monsoon precipitation, inferred from absolute-dated and instrumentally calibrated speleothem oxygen isotope records from regions (central and northeast India) that have diametric responses to active-break monsoon circulation patterns. On centennial timescales (AD 1400-2008), precipitation variability from these two regions exhibit opposing behavior, oscillating between periods with a persistently "active-dominated" (AD ~1700 to 2007) and a "break-dominated" (AD 1400 to ~1700) regime. The switch between these regimes occurs abruptly (within decades) at a time (AD ~1650-1700) when a proxy record of upwelling intensity from the Arabian Sea suggest an abrupt increase in the monsoon winds. On the basis of these observations, we hypothesize that the frequency distribution of active-break periods varies on centennial timescales, implying a leading role of internal dynamics in governing the ISM response to slowly-evolving changes in the external boundary conditions.
description: Stalagmite oxygen isotope (d18O) data from two caves located 
in central and northeast India, plus a composite Core Monsoon 
Zone stalagmite d18O record, composed of Jhumar Cave d18O data 
combined with previously published Dandak Cave d18O data. 

The instrumentally calibrated stalagmite oxygen isotope record 
(JHU-1) from Jhumar Cave in Central India provides a millennial 
length (AD 1075 to 2007) history of monsoon rainfall variations 
over the core monsoon zone of India (CMZ).  The record contains 
642 oxygen isotope measurements with an average temporal 
resolution of 1.45 years and its chronology is established by 
counting annual growth layers.  A maximum uncertainty of ~10 
years at the bottom of the stalagmite is inferred from multiple 
counts of growth laminae.  A suite of 230Th dates were also 
obtained to test the annual nature of the laminations (Table S2). 
While all 230Th ages fall on or near the slope that describe the 
laminae-based age model, we did not use 230Th dates in constructing 
the age model because of high detrital thorium of JHU-1 stalagmite, 
which produced substantially large error in 230Th dates 
(2 sigma > 200 years). 

The multicentennial-length (AD 1399 to 2007) speleothem d18O record 
(WS-B) from Wah Shikar Cave in northeast India is established by 
625 d18O measurements with an average temporal resolution of 0.97 
years (Table S1). The record provides a history of monsoon rainfall 
variations over northeast India (NEI). The age model for the WS-B 
stalagmite is developed from seven 230Th dates, which are in correct 
stratigraphic order, with 2 sigma error ranging from 28 to 70 years 
(Table S2). Four additional 230Th dates produced unreasonably high 
error (> 100 years) and were subsequently discarded. The age model 
is developed by linearly interpolating between individual dates 
except for the segment between the bottom two dates where we 
calculated the age at the bottom of the stalagmite by extrapolating 
the age-depth curve from two prior better-constrained dates. 

The Jhumar Cave stalagmite d18O data provides a history of monsoon 
rainfall variations over the core monsoon zone of India (CMZ). 
The record complements our previously reported millennial-length 
(AD 625 to 1562), absolute-dated oxygen isotope record (DAN-D) 
from Dandak Cave (Berkelhammer et al., 2009; Sinha et al, 2007), 
which is located ~20 km west of Jhumar Cave. Both oxygen isotope 
time series were linearly interpolated to an annual timescale 
and averaged (over the period of overlap) to produce a composite 
oxygen isotope record that provides a continuous history of the 
CMZ's monsoon rainfall variations over the past ~1400 years 
(AD 2007-625) (Table S1). The composite record captures nearly 
every major drought during the instrumental period as well as 
historic failures of monsoon, including extended intervals of 
substantially weaker monsoon during the 14th and 17th century. 
The latter are independently corroborated by historical accounts 
and other regional proxy records. 

Site Locations:
Jhumar Cave, Central India: 18°52'N, 81°52'E, 600 masl 
Wah Shikar Cave, Northeast India: 25°15'N, 91°52'E, 1290 masl 
Dandak Cave: 19°N, 82°E, 400 masl