Interglacial theme and variations: 500 k.y. of orbital forcing and associated responses from the terrestrial and marine biosphere, U.S. Pacific Northwest Mitchell Lyle Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, MS 1536, 1910 University Drive, Boise, Idaho 83725-1536, USA
Linda Heusser Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
Timothy Herbert Department of Geological Sciences, Brown University, Box 1846, Providence, Rhode Island 02912, USA
Alan Mix College of Ocean and Atmospheric Science, 104 Oceanography Administration Building, Oregon State University, Corvallis, Oregon 97331-5503, USA
John Barron U.S. Geological Survey, MS 915, 345 Middlefield Road, Menlo Park, California 94025, USA
ABSTRACT Sediments collected off northern California by Ocean Drilling Program Leg 167 contain time series that show strong, orbitally driven insolation forcing of surface oceanographic conditions. Orbital forcing caused a strong response in the distribution of major terrestrial vegetation but a less predictable response for primary productivity offshore. Terrestrial vegetation responded primarily to regional sea surface temperature (SST). Coastal ocean productivity appears highest when SST is moderately high, not during peak interglacial conditions nor during insolation maxima. When individual interglacial intervals are examined closely, each has a different signature. Two of six interglacials (MIS [marine isotope stage] 5 and MIS 11) have higher SST than modern conditions, but each elicits a different response from the terrestrial and marine communities. The type of vegetation and the strength of upwelling vary between interglacials, depending on the relative strength of factors that drive the warming, including insolation, ice-cap size, and level of greenhouse gases. Keywords: paleoceanography, paleoclimate, Pleistocene, pollen, paleoproductivity, sea surface temperature, vegetation, alkenones, ODP Site 1020, Pacific Northwest, western North America.
INTRODUCTION The evolution of the atmosphere, oceans, and vegetation since the Last Glacial Maximum (;21 ka) has been well documented in the U.S. Pacific Northwest (Moore, 1973; Heusser and Shackleton, 1979; Barnosky et al., 1987; Lyle et al., 1992; Gardner et al., 1997; Mix et al., 1999), but surprisingly little is known about the rest of the Pleistocene. Because of high sedimentation rates offshore, sediment cores of a sufficient length to span even the past 150 k.y. are rarely collected. Drilling on Ocean Drilling Program Leg 167 allowed recovery of high-resolution sediment sections from the Pliocene to Holocene. We report here on the biogenic response to the last six glacial-interglacial cycles. Site 1020 (Fig. 1), located 170 km west of the coast of California, is situated ;600 km south of the maximum Cordilleran ice-sheet advance. The sediments contain sufficient foraminifera to produce an oxygen isotope stra-
tigraphy and have a pollen record from the coastal plant communities of northern California (Fig. 2; Heusser et al., 2000). They contain a variable amount of biogenic CaCO3 (;1%–30%), have relatively high Corg contents, and are well suited for biomarker studies (Kreitz et al., 2000; Lyle et al., 2000). The sedimentation rate is ;10 cm/k.y. Because all analyses were made from the same sediments and for the most part on the same samples, the offsets between the different records are real despite any error in the age model. OCEAN-CLIMATE THEME The regional state of the oceans, as represented by alkenone-estimated sea surface temperature (SST), strongly affects the types of coastal vegetation (Fig. 2). The lowest SSTs lead ice-volume maxima, and warming begins ;12 k.y. prior to deglaciation (Herbert et al., 2001). The same lead is found in the abundance of cold-tolerant vegetation, as illustrat-
ed by Artemisia pollen (sagebrush and related species). Maximum SSTs either coincide with or are slightly younger than the ice-volume minima. The abundances of warm-tolerant vegetation peak with SSTs. Canonical correlation analysis of late Pleistocene records in the northeastern Pacific (Pisias et al., 2001) found that changes in the radiolarian assemblages are highly correlated with pollen changes from adjacent North America, implying a direct link between offshore ocean conditions and onshore climate. The coincident response should make it possible to develop a high-resolution stratigraphy across the shoreline beyond the range of radiocarbon dating. We used a varimax factor analysis to group pollen data into three assemblages. Vegetation along the northwest coast of North America is closely associated with climatic gradients, and it is a sensitive index of temperature and moisture. Systematic variation in the prominence of the vegetation assemblages downcore re-
q 2001 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; December 2001; v. 29; no. 12; p. 1115–1118; 4 figures.
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Figure 1. Location map of Ocean Drilling Program (ODP) Site 1020 relative to modern coastal redwood (light shading) and western hemlock (dark shading) (Little, 1971). Site was drilled on ODP Leg 167 (Lyle et al., 1997).
flects clear variation of climatic conditions that did not exceed, at least for extended periods, the tolerances of taxa in these communities. Our study indicates that these communities have been present in the Pacific Northwest for the past 500 k.y.; reconnaissance finds evidence of similar vegetation since the earliest Pliocene (ca. 5 Ma, Axelrod, 1977; Heusser, 2000). The pollen assemblages represent vegetation types dominated by (1) coastal redwood (Sequoia sempervirens), oak (Quercus spp.), and alder (Alnus spp.), (2) western hemlock (Tsuga heterophylla) and Sitka spruce (Picea stichensis), and (3) pine woodland, chaparral, and grass prairie, a mosaic not unlike that of plant communities along the northern California and southern Oregon coasts today. Coastal redwood, oak, and alder (represented by Sequoia in Fig. 2) are in high abundance in marine isotope stage (MIS) 1 and are abundant only during strong interglacials, or only ;10% of the total time span. Sequoia was nearly absent from the coastal plant community during glacials. The coastal redwood forest is not displaced southward (Heusser, 1998); instead, relatively few trees must have
survived within the modern range and reproduced vegetatively for extended periods of time. The long-lived habit (;1000 yr) of coastal redwood requires only very low replacement rates, which can be met by sprouting. The western hemlock–Sitka spruce assemblage is representative of Holocene forests that thrive near the U.S.-Canada border, but extend south almost to San Francisco (Fig. 1). These forests need significant winter precipitation to grow well, but they tolerate higher mean summer temperatures (;20 8C) and colder minimum temperatures than the coastal redwood forest (Heusser, 1985). At Site 1020 we observe the greatest abundance of this forest type during weak interglacials and during deglaciations (Fig. 2, as illustrated by western hemlock). It has also become a more prominent forest type in northern California since ca. 350 ka. In the last deglaciation (MIS 2-1 boundary), the spruce-hemlock community peaked early in the deglaciation, ca. 15 ka. The pine-scrub-herb assemblage is composed of communities that tolerate greater moisture stress and temperature extremes than the two conifer assemblages. It is a typical
Figure 2. Benthic d18O, CaCO3, Corg, and representative pollen species profiles compared to alkenone-estimated sea surface temperature (SST). Interglacial periods are shaded and identified by their marine isotope stage (MIS). Ages are from Shackleton (2000) for ,400 ka and Imbrie et al. (1984) for .400 ka. Pollen associated with strong interglacials, e.g., Sequoia, are prominent for only ~10% of record. Pollen of cold-climate vegetation, e.g., Artemisia spp., are associated with coldest SSTs. Intermediate conditions cause a third group, e.g., Tsuga heterophylla, to be prominent. ODP is Ocean Drilling Program; rmcd is revised meters composite depth. 1116
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Figure 3. Comparison of pollen time series of alder, oak, and redwood with summer insolation at 658N. Redwood is most abundant in interglacials with weak precessional forcing, and oak abundance is highest when summer insolation is high and sea surface temperature is warmest. Alder, a species that rapidly colonizes disturbed ground, reaches maxima during deglaciations.
glacial assemblage throughout the Pacific Northwest (Mehringer, 1985), and represents open pine forest conditions. This assemblage, represented in Figure 2 by Artemisia pollen, is typical of coldest conditions. The pine-herbscrub assemblage is highest when the SST at Site 1020 is lowest. Responses to strong interglacial conditions give perhaps the best indicator of potential responses to global warming. The trio of alder, redwood, and oak (Fig. 3) typify interglacials for the past 500 k.y. At Site 1020, alder, an opportunistic species, typically reaches its peak abundance during deglaciation, whereas oak peaks when the SST is highest. Sequoia
Figure 4. Paleoproductivity indicators shown for 0–140 ka at Site 1020. Benthic d18O profile (top record) defines marine isotope stages (MIS). Last strong interglacial (MIS 5e) does not have high primary productivity, in contrast with Holocene (MIS 1). At MIS 5e, diatoms, measured as the number observed per slide traverse (bottom, dashed lines), are essentially missing from the sediments, and Corg mass accumulation rate (MAR), middle, is low.
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peaks are displaced to slightly later in the cycle. Redwood forests are best developed where fog associated with cold, upwelling waters moderates summer temperatures (July mean of 17 8C) and ameliorates drought. Oak better tolerates dry hot summers and tends to peak under warmer and drier conditions than redwood. INTERGLACIAL VARIATIONS Terrestrial Vegetation The highest abundance of Sequoia pollen is found in MIS 11 and the Holocene, interglacials marked by a weak amplitude within the precessional band of summer insolation at
658N. Oak is most abundant in MIS 5e, an interglacial where high eccentricity has magnified the precession cycle (Fig. 3). MIS 5 and 7, both interglacials with high-amplitude precession cycles, are the only intervals in which the percentage of oak pollen exceeds that of Sequoia. The temporal lead of oak over redwood and the higher abundance of oak in these short precession-driven cycles also imply that oak propagates more rapidly than Sequoia when conditions warm and may outcompete when climatic optima are brief. We compared the pollen abundance with alkenone-derived SSTs (Kreitz et al., 2000). Oak becomes abundant after SST warms above ;8 8C and remains high at the highest SST. In contrast, redwood pollen becomes abundant only after SST surpasses 10 8C. In MIS 11, redwood pollen dropped when SST rose beyond ;13 8C, suggesting environmental stress. In MIS 5e, highest redwood abundance occurred after SST cooled below 13 8C. Western hemlock, in contrast, is strongly represented in the weaker interglacial and glacial intervals but is rare during strong glacials or strong interglacials. In the site 1020 record, its abundance reaches maxima when SSTs are ;9 8C. Marine Productivity There is no consistent glacial-interglacial signal for orbital forcing of marine productivity. The doubling in productivity associated with the MIS 1–2 deglaciation (Lyle et al., 1992; Dean and Gardner, 1998) is not typical of earlier interglacial regimes. Measures of productivity at MIS 5e (Fig. 4), for example, suggest that upwelling was poorly developed and that productivity was significantly less than the modern amount (Poore et al., 2000). Higher productivity occurred later in MIS 5, but not at the minimum ice volume. Figure 4 shows two records, the Corg mass accumulation rate (Lyle et al., 2000), and the number of diatom tests in samples from MIS 5 (Poore et al., 2000). Of the diatom assemblage, ;90% consists of two upwelling forms, Thallasionema nitzschioides, an offshore upwelling and/or spring bloom species (Sancetta, 1992; Sancetta et al., 1992), and Chaetoceros spores, a coastal upwelling indicator. Very few diatoms were found in the sediments deposited during MIS 5e, in contrast to later in MIS 5. The lack of diatoms indicates that upwelling strength was low or extremely restricted to the immediate coastal region. Similarly, the Corg mass accumulation rate during MIS 5e is lower than that of the Last Glacial Maximum, a low-productivity interval (Lyle et al., 1992; Dean and Gardner, 1998). Additionally, the presence of subtropical diatoms and warmwater foraminifera in MIS 5e implies that conditions may have resembled those during El 1117
Nin˜o years of the Holocene (Poore et al., 2000). MIS 11 is the best analogue to the Holocene among the recent interglacial periods in terms of insolation conditions and greenhouse-gas levels (Berger and Loutre, 1999), but it appears to have had significantly lower productivity than the Holocene, on the basis of Corg mass accumulation rates and opal contents (Kuroda et al., 2000). High productivity does not seem to be characteristic of strong interglacials. Instead, the Holocene appears unique in terms of its high productivity relative to warm SSTs or minima in ice volume. SEDIMENTARY RECORD OF WARM CONDITIONS Two interglacials in the past 500 k.y., MIS 5e and MIS 11, have produced SSTs ;2 8C higher than in the Holocene at Site 1020. In MIS 5e, oak partly replaces coastal redwood, and coastal upwelling appears weak. In contrast, MIS 11 has relatively little oak, but coastal redwood was also suppressed at the highest SSTs. Low Corg mass accumulation rates imply that coastal upwelling was weak. The coastal redwood is partly replaced by pine and Sitka spruce rather than by oak during the period of highest SSTs. Rapid changes in summer insolation around MIS 5e probably promoted oak over coastal redwood along the coast. An abundance of oak implies that summer temperatures were significantly warmer than in the Holocene, whereas winter rainfall may have been lower. In MIS 11, increases in pine, western hemlock, and Sitka spruce, all species that can tolerate somewhat warmer summer temperatures than coastal redwood but are not dependent upon coastal fog, imply somewhat warmer, less foggy summers. Because western hemlock and Sitka spruce need high winter precipitation, it appears that northern California had more rainfall during the winter season. Records from both MIS 5 and MIS 11 thus suggest that coastal redwood could be stressed by warming in the next century. SSTs of the northeastern Pacific must warm significantly before conditions exceed the temperature-precipitation range that modern coastal forests have already survived, however. Although coastal redwoods will be more stressed than earlier in the Holocene, climate conditions should not be far from their optimum. If SSTs do warm and reduce the landsea temperature contrast, there may be some weakening of the upwelling system similar to conditions in MIS 5e and perhaps MIS 11. ACKNOWLEDGMENTS This research was supported by National Science Foundation grant OCE-9811272 (Lyle and Heusser). We also greatly appreciate the work by all the ODP Leg 167 shipboard party (scientific and crew) who recovered the core and ran the initial analyses. 1118
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