The Last 1200 Years of Rainfall/Runoff Variability along the Central Mexico Pacific Coast Associated with the North American Monsoon

William Berelson,Emily Mortazavi,Caitlin Tems,Ellen Platzman,Steve Lund,Yvonne Hamann

doi:10.3390/oceans2030030

Abstract

This study presents new evidence for long-term variability in the late Holocene North American Monsoon (NAM), Pacific coast of Mexico. We have carried out a rock magnetic study on two deep-sea sediment cores from the Pacific coast Pescadero Basin. The magnetic intensities estimate total magnetic material and are a proxy for total clastic sediment. Ratios of magnetic intensities estimate the grain size of magnetic material. The rock magnetic data show a decimeter scale, multi-decadal oscillation with fourteen cycles (A-N) over the last 1200 years. These oscillations reflect alternating intervals of stronger/coarser magnetic/clastic flux to the coastal ocean and intervals of weaker/finer magnetic flux. We think these variations are caused by variations in long-term dominance of the NAM; summer (wet) monsoons produce rainy conditions (with runoff) while winter (dry) monsoons produce significant offshore winds, increased upwelling/biological productivity. We can correlate our variability to two other published studies southeast of Pescadero Basin, coastal lake sediments in Laguna de Juanacatlan and a Juxtlahuaca Cave stalagmite. Both of these studies estimate local rainfall. We see evidence of the same pattern of multi-decadal rainfall-runoff variability in these records as we see in Pescadero Basin, which is synchronous to within ±25 years over the last 1200 years. The multi-dacadal pattern of hydrologic variability in all three records varies in cycle duration from ~90-years wet/dry cycles in the Little Ice Age (1400–1850 AD) to ~60-years cycles in the Medieval Climate Optimum (1100–1400 AD). This variability in cycle duration suggests some chaotic nature to the regional NAM climate pattern or some long-term non-linear forcing (PDO?).

Highlights

Tems et al [1] analysed deep-sea sediment cores from the Pescadero Basin, 616 m water depth, adjacent to the Pacific coast of central Mexico (Figure 1), to look for oceanographic evidence of oxygen minimum zone (OMZ) and productivity variations over the last 1200 years
Tems et al [1] hypothesized that longer-term variability in the North American Monsoon (NAM), due to a changing ratio of winter versus summer dominance in the NAM, caused their observed pattern of OMZ/productivity variability and might be due to multi-decadal- to centennial-scale Pacific Decadal Oscillations (PDO; [5,6,7])
Sediment cores from Pescadero Basin (Table 1) were studied rock magnetically to assess the concentration of magnetic particles in the sediments, the magnetic grain size distribution, and variations in the percentage of various magnetic minerals

Summary

Introduction

Tems et al [1] analysed deep-sea sediment cores from the Pescadero Basin, 616 m water depth, adjacent to the Pacific coast of central Mexico (Figure 1), to look for oceanographic evidence of oxygen minimum zone (OMZ) and productivity variations over the last 1200 years. They suggested that long-term variability in the North American Monsoon (NAM) might be the cause of their observed variability. Tems et al [1] hypothesized that longer-term variability in the NAM, due to a changing ratio of winter versus summer dominance in the NAM, caused their observed pattern of OMZ/productivity variability and might be due to multi-decadal- to centennial-scale Pacific Decadal Oscillations (PDO; [5,6,7]).

Methods

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Discussion

Conclusion