DOI: 10.25921/xrx1-2k25
title: NOAA/WDS Paleoclimatology - Makassar Strait 2000 Year Foraminiferal Mg/Ca and Stable Isotope Data

description: The Indonesian Throughflow (ITF) is the only low-latitude conduit for exchange of surface and thermocline waters between the Indian and Pacific Oceans; 80% of the ITF waters pass through the Makassar Strait. The flux of heat and salt through the strait is, in part, a function of the density difference between its southern and northern ends. Here the Mg/Ca and stable oxygen isotope compositions of the planktonic foraminifer Globigerinoides ruber sampled from two sediment cores collected at the northern and southern ends of the Makassar Strait are used to reconstruct surface-water temperature (SST), sea-surface salinity (SSS), and seawater density variability over the past 2000 yr. Maximum SST and SSS occurred at both sites between 850 and 700 yr ago, coinciding with the Medieval Solar Maximum and Medieval Warm Period (ca. 1000-700 yr ago). SST and SSS declined at both locations after 700 yr ago and reached minimum values during the Little Ice Age, between 300 and 100 yr ago. However, the magnitude of SST and SSS change over the past 2000 yr was distinctly different between the two sites, especially during the period between 2000 and 900 yr ago. This led to changes in the south to north density gradient within the Makassar Strait. Specifically, the time interval from ca. 2000 to 800 yr ago was marked predominantly by higher densities at the northern end relative to the southern end of the strait, suggesting reduced heat flux from the Pacific to the Indian Ocean. This is consistent with previous work showing that this time interval was also marked by frequent El Niño conditions. Conversely, the period from ca. 800 to 300 yr ago, including the LIA, was marked by frequent periods when surface density was higher at the southern end of the strait, indicative of enhanced heat flux to the Indian Ocean.
description: Mg/Ca and stable oxygen isotope compositions of the planktonic 
foraminifer Globigerinoides ruber over the past 2000 years, 
sampled from two sediment cores collected at the northern 
and southern ends of the Makassar Strait, Indonesia. 

Sample preparation: Samples were freeze-dried, then soaked in 
a Calgon-water solution, before being washed through a 63um sieve.  
Samples were dried and the surface dwelling G. ruber were picked 
from the 250 to 350 um size fraction. 

Trace metal analysis: All samples used for trace metal analyses 
underwent the rigorous cleaning procedure of Boyle (1981) to 
eliminate clay and organic matter contamination, although the 
reductive cleaning step was not used.  Calcium and magnesium 
were measured simultaneously using a Jobin Yvon Ultima 
Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES).  
Based on repeated analysis of standards, the 1S analytical precision 
is better than 1.0%. 

Stable isotope analysis:  All stable isotope measurements were made 
using a VG Optima stable isotope ratio mass spectrometer equipped 
with an automated carbonate system.  All d18O values are reported 
in delta notation relative to the Vienna Pee Dee Belemnite (VPDB) 
standard.  The long-term standard reproducibility for oxygen isotopes 
based on replicate analyses of a reference standard is ±0.07%. 

Age model: Radiocarbon analyses were performed at the Center for 
Atomic Mass Spectrometry at Lawrence Livermore National Laboratory 
on mixed samples of G. ruber and G. sacculifer from the >150 um size 
fraction.  The calendar ages listed were calibrated using the Calib 5.0 
program (Stuiver and Riemer, 2003), with an additional reservoir age 
correction of 75±80 years (Southon et al., 2002). 

Core MD98-2160: 05°12.07'S, 117°29.20'E, water depth 1185 m 
Core MD98-2177: 01°24.20'N, 119°04.68'E, water depth 968 m