PALEOCEANOGRAPHY, VOL. 10, NO. 2, PAGES 311-325, APRIL 1995

Earliest Oligocene increase in South Atlantic productivity as interpreted from "rock magnetics" at Deep Sea Drilling Project Site 522

Paul Hartl, Lisa Tauxe, and Timothy Herbert Scripps Institution of Oceanography, La Jolla, California

Abstract. The magnetic properties of the sediments ("rock magnetics") at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the dif- ference to a slightly greater proportion of the finest grained, so-called "superparamagnetic" mag- netite in the Eocene sediments. We believe that the lower proportion of very fine-grained mag- netite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene-to-Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine-grained magnetite component. Also parallel to these is a trend toward heavier 8•3C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain-size fraction of magnet- ite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of en- vironmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.

Introduction

The global climate change from the "greenhouse" of the mid- dle Eocene to the "icehouse" of the early Oligocene has been called "the most profound climatic event of the Tertiary" [Wolfe, 1978]. In the terrestrial realm the transition is marked by an equatorward migration of tropical and subtropical broadleaf ev- ergreen forests and their replacement by temperate broadleaf de- ciduous forests. Wolfe [1978] interpreted this change as indicat- ing a 12øC - 13øC decline in mean annual temperatures at 60 ø lati- tude, with a strongly increased annual temperature range from Eocene values of 3ø-5øC to Oligocene values of 21øC.

The oceans also felt the change as marine floras and faunas shifted from high biotic diversity to lower diversity "oligotaxic" ecologies. Although much change was gradual in response to a long-term cooling over the middle and late Eocene, there is also strong evidence of a far more rapid cooling "step" in the •arliest Oligocene. This step is evident as one of the largest increases in •O of the Tertiary, indicating a significant global cooling of ocean waters [Shackleton and Kennett, 1975, Kennett and Shackleton, 1976; Keigwin, 1980; Kennett and Barker, 1990; Zachos et al., 1993] or the beginning of significant Antarctic ice

I Also at Fort Hoofddijk Paleomagnetic Lab, Utrecht, Netherlands.

Copyright 1995 by the American Geophysical Union.

Paper number 94PA03150. 0883- 8305/95/94PA-03150510.00

build-up [Matthews and Poore, 1980]. In all likelihood this step is a product of both effects and marks the initiation of intermit- tent continental glaciation of Antarctica [Miller et al., 1991a]. The boundary also marks a dramatic deepening of the calcite compensation depth (CCD), particularly in the Pacific, but also in the Indian and Atlantic Oceans [Van Andel, 1975]. Rapid marine biotic changes near the Eocene/Oligocene boundary include a mass extinction of large benthic foraminifera from shallow water carbonate facies [Adams et al., 1986], a large scale turnover and reduction in diversity of planktonic foraminifera [Boersma and Premoli-Silva, 1986], significant radiolaria extinctions [Reidel and Sanfilippo, 1986] and a reduction in the diversity of calcare- ous nannofossils [Wei and Wise, 1990]. Hiatuses are common near the boundary and are thought to have resulted from an in- crease in thermohaline circulation and the initiation of cold Ant-

arctic bottom water currents in the earliest Oligocene [Shackleton and Kennett, 1975; Kennett and Barker, 1990].

The overall gradual climatic transition from the warmth of the early Eocene to the relatively chilly early Oligocene is thought to have been driven by plate tectonic changes, particularly by openings of the Drake Passage and the Tasman Sea, and by the closure of Tethys, which resulted in a major reorganization of ocean circulation patterns and the isolation of Antarctica [Shackleton and Kennett, 1975, Kennett and, Shackleton, 1976; Kennett and Barker, 1990]. The more rapid steplike cooling events are thought to mark thresholds, which result from nonlin- ear positive feedbacks, such as the rapid albedo change over the Antarctic continent and surrounding waters with the initiation of

312 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

permanent snow cover and sea ice formation [Kennett and, Shackleton, 1976; Kennett and Barker, 1990; Zachos et al., 1993].

A major change in climatic regime can exert profound influ- ences on the source, transport, and depositional environment of accumulating pelagic sediments. The magnetic properties of the sediments ("rock magnetics") can also reflect such variations be- cause rock magnetics are a function of the mineralogy, concen- tration, and grain size of the magnetic carrier, any of which could be influenced by environmental changes. Therefore, rock mag- netic properties are capable of recording time series information of climatic and oceanographic changes within accumulating sediment columns. Furthermore, because of the rapid and nondestructive nature of most magnetic measurements, high resolution records of considerable length are possible. Examples include Bloernendahl et al. [1988], who used various rock mag- netic parameters to track glacial/interglacial variations in mag- netic mineralogy, concentration, and grain size in equatorial At- lantic cores off the Saharan Coast, matching these fluctuations to the "SPECMAP" oxygen isotope record of Irnbrie et al. [1984], Mead et al. [1986] used down core variations in magnetic sus- ceptibility to track Oligocene changes in sediment accumulation rates as well as Milankovitch cyclicity. Many other studies have used rock magnetics to help decipher variations in paleoenviron- mental conditions (see work by King and Channell [1991] for a review).

Deep Sea Drilling Project (DSDP) Site 522 yielded an out- standing and continuous record of the Eocene/Oligocene bound- ary. Core recovery was excellent [Hsii et al., 1984], the magnetic properties of the sediments are ideal for magnetostratigraphy and rock magnetic analysis [Tauxe et al., 1984; Hartl et al., 1993]; the nannofossil and foraminiferal records contain all important index species and assemblages [Percival, 1984; Poore, 1984]. No hiatuses are evident across the boundary [Hsa et al., 1984] and, very importantly, core breaks did not obscure the boundary itself. The original shipboard paleomagnetic sampling of this core was done at 20 cm intervals. We have improved this reso- lution approximately fivefold by resampling at intervals of 3-4 cm. In this paper we describe, compare, and attempt to relate the results of our rock magnetic and carbonate analyses of Site 522 sediments to previous biostratigraphic, isotopic, and paleocean- ographic studies of the Eocene/Oligocene boundary from this and other locations.

Site Location and Measurement Methods

DSDP Site 522 is located in the South Atlantic at 26øS, 5øW at a depth of 4400 m. At the time of the Eocene/Oligocene bound- ary, the Site's latitude was about 33øS [Tauxe et al., 1983] and the water depth was approximately 3000 m. The sediments from this period are largely nannofossil oozes, averaging approxi- mately 88% carbonate, with a largely clay secondary component that contains minor nonopaline quartz. The sediment accumula- tion rate for this period was about 1 cm kyr '• [Hsii et al., 1984; Mead et al., 1986]. We sampled the upper Eocene to lower Mio- cene at intervals of 3-4 cm and measured magnetic properties of 2600 individual specimens and the carbonate content of 300.

Natural remanent magnetization (NRM), anhysteretic rema- nent magnetization (ARM), saturation isothermal remanent mag- netization (SIRM), low-field magnetic susceptibility (Z), dry

weight, and percent CaCO3 were all measured using facilities at the Scripps Institution of Oceanography, La Jolla, California. ARM was produced in a 0.1-mT bias field and an 85-mT alternat- ing field (AF). SIRM was induced with a 0.6 T impulse field. NRM, ARM and SIRM were all measured with a three-axis CTF cryogenic magnetometer housed in a magnetically shielded room at Scripps. Susceptibility (Z), was measured on a Kappa Bridge KLY-2.02 susceptibility bridge. Percent CaCO3 was measured using a Coulometrics CO2 coulometer. Frequency dependence of susceptibility was measured with a Lakeshores susceptometer at the Institute for Rock Magnetism at the University of Minnesota, Minneapolis, Minnesota where we also used a Quantum Design SQUID susceptometer for measurement of low temperature hys- teresis parameters and Princeton Instruments "MicroMag" alter- nating gradient field magnetometer for measuring room tempera- ture hysteresis properties.

Rock Magnetics as a Useful Paleoceanographic Indicator

The magnetic properties of sediments are controlled by the mineralogy, concentration, and grain size of the magnetic car- der(s) [e.g., O'Reilly, 1984]. Thus in principle they could pro- vide information on these parameters if the controls on magnetic behavior are well known. There is a bewildering variety of mag- netic measurements that can be made. Some of these, such as

magnetic susceptibility (•), isothermal remanence (IRM) and an- hysteretic remanence (ARM) are particularly well suited to the purposes of so-called "environmental magnetism" because they are readily measurable and are reasonably well understood from a theoretical and experimental point of view.

Magnetic susceptibility is obtained by subjecting a specimen to a magnetic field (B) and measuring the magnetic moment in- duced (M). When the applied field is small, there is a reversible, linear relationship described by M/B which is the initial magnetic susceptibility (Zi, here just •). This response is the result of un- paired electronic spins, so practically all substances display mag- netic susceptibility, including nonmagnetic phases such as car- bonate or clay; these show diamagnetic or paramagnetic suscep- tibility, respectively, as opposed to the ferromagnetic susceptibil- ity of magnetic phases. Thus • is controlled by properties of the magnetic phases and also by any diamagnetism and/or paramag- netism contributed by the matrix material. However, the re- sponse of ferromagnetic phases such as magnetite is orders of magnitude stronger than that of the matrix material so that in many geologic materials, Z is overwhelmingly dominated by the magnetic fraction. In ferromagnetic phases the ability of elec- tronic spins to rotate in response to an applied field is compli- cated by several factors. First, in a ferromagnetic phase, the spins are acting in a coordinated way, which is responsible for the very large moments achievable. Second, the spins have preferred di- rections within the grains as a result of shape, stress, or crystallo- graphic anisotropy. In very small grains the various anisotropy energies are trivial compared to the thermal energy and provide no constraints. These grains possess no long-term magnetic memory and respond to magnetic fields in much the same way as paramagnetic phases, but because of the coordinated action of the spins, the response is much stronger, hence the name "superparamagnetism" (SPM). In the superparamagnetic range, • is a linear function of grain volume, and hence varies with the cube of the diameter (see Figure 1 a).

HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS 3 I3

At some critical grain diameter, anisotropy energies become significant, and the spins can no longer freely rotate, but are re- stricted to several preferred directions separated by a large anisot- ropy energy barter. The grains are uniformly magnetized and are called "single domain" (SD). SD grains require a large external field to coerce the domain direction to reverse itself, and there- fore are termed magnetically "hard", meaning that SD grains ex- hibit comparatively low susceptibility, high stability (high coer- civity) and high remanence moment. The magnetic susceptibility is consequently much lower than in the SPM region (there is a factor of 25 reduction in % at the upper limit of SPM behavior (see Figure 1 a).

As grain diameter increases further, the energy of the external magnetic field becomes so large that it is energetically favorable for the grain to organize spins into regions (domains) with differ- ent directions, thereby lowering the energy of the extemal mag- netic field. Such grains are called multidomain (MD) grains. Domain "walls", which are zones of highly structured transitional

orientation, separate domains from each other. These walls are relatively easily moved about by outside magnetic fields, result- ing in new domain configurations and a new net magnetic mo- ment for the grain as a whole. MD grains are thus termed mag- netically "soft", meaning that they are easily magnetized and de- magnetized. MD grain domain walls also shift about respon- sively to a susceptometer's small oscillating magnetic field, pro- ducing a relatively large induced moment for a given external field, that is, such specimens exhibit enhanced magnetic suscep- tibility (up to several times that of an SD grain, see Figure la). Between the SD and MD grain sizes lies the "pseudo-single do- main" (PSD) grain size range, in which smaller MD grains with just a few domains exhibit magnetic behavior more akin to SD than MD. This is a very common size range for magnetite, and many of the best paleomagnetic records are carded by PSD mag- netite. Please note that by far the largest effect on % is the change from SPM to SD behavior and the change from SD to MD is a relatively minor effect.

3O

,,o 25

x

'-' 20 7 ,,'15

E < 10

• 5

SPM SD PSD MD I I , I x

/• + Hartstra (1982) - x /. %ARM [] King et al. (1982)

/x .. \ • Ozdemir and Banerjee (1982) - x x Maher (1988) _

10-2 10-1 10 o 10 • 102

3O

2O

15

10

3O

,-, 25

• 10 E

10-2 10-•

ß Dankers (1978) • Ozdem:r and Banerjee (1982) x Maher (1988)

10 o

grain diameter, •m

i , ,-•, ,, -- • I--- •-i- I ---I •1 •11 •

10 • 102

Figure 1. (a) Magnetic susceptibility (%), normalized to its value at 101.tm. The dashed section is the calculated peak resultant from superparamagnetic behavior. The mismatch with the data of Maher [1988] in this region is likely due to experimental difficulties associated with grain size control and dispersion of synthetic magnetite grains at these very small sizes. (b) "Rock magnetic" parameters isothermal remanent magnetization (SIRM) and anhysteretic remanent magnetization (ARM) versus grain size. To accommodate ready comparison, each has been normalized by its value at 101.tm and thus the unusual scaling factors.

314 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

The exact threshold sizes for SPMISD, SD/PSD and PSD/MD

are very poorly constrained. The most quoted size for the SPM/SD threshold is 0.03 I. tm [Dunlop, 1973], but Pick and Tauxe [1994] have argued that this is probably too large. In Fig- ure lb we have arbitrarily chosen a threshold size of 0.025 I. tm for the theoretical curve (dashed line).. We also use 0.08 !.tm for the SD/PSD boundal'y and 10 gm for the PSD/MD boundary [Butler and Banerjee, 1975; Thompson and O!dfieM, 1986; Wil- liams and Dunlop, 1988]. Because of difficulty in dispersing magnetite, determining actual sizes and other experimental prob- lems, as well as the fact that the true SPM/SD threshold size is

poorly constrained, the measured Z values do not fit the theoreti- cal curve very well in the SPM region (Figure l a). However, what is important here is not the exact threshold size, but that SPM grains will have greatly enhanced Z values relative to their closely related sister SD grains and that the threshold is quite abrupt.

We plot experimentally determined values of ARM and satu- ration IRM (SIRM) as a function of grain diameter and domain state (Figure lb). True SPM grains will, by definition, have no remanence. SD grains, because of their uniform magnetization have enhanced remanences and the net moment decreases as new

domains are created with increasing grain size. To allow for ready comparison, we have normalized both to their values at 10 gin, whence the unusual scaling factors. Note that the ARM data, which were gathered using synthetic magnetite, are poorly described by the line. There are many very difficult problems as- sociated with using synthetic magnetites, primarily associated with dispersal of the highly magnetic grains to resemble natural concentrations and the tendency for small grains to cling to larger grains, greatly altering magnetic response. Unfortunately, this is the current state of the data. However, despite these problems, it is clear that ARM is far more selectively responsive to fine grain sizes than SIRM. The sudden increase by a factor of 5 or so in the range from 0.1 Bm to 0.06 g m followed by complete loss of remanence at the SD/SPM boundary means that ARM will be far more sensitive to grain size changes over this limited range than SIRM. SIRM, on the other hand, varies much less with grain size in this range, until it too goes to zero as the SD/SPM bound- ary is approached.

It is clear from the data in Figure 1 that the ratios of ARM/Z and SIRM/Z will be exceedingly sensitive to changes over a very small grain size range particularly near the SPM/SD threshold. In this grain size range the ratio ARM/z will be sensitive both in the ARM and Z terms and may yield ambiguous results for grain size distributions centered near the ARM maximum. SIRM/z will be dominated by changes in Z and would be expected to yield relatively unambiguous grain size information.

Based on the information summarized in Figure 1, magnetic grain size information can be gleaned from the various rock mag- netic parameters. The SPM/SD threshold, for example, should be readily identifiable. A grain is defined as being superparamag- netic when its magnetic "memory" is shorter than a specified laboratory timescale, say, 102 or 103 s, at laboratory temperatures, say, 300 K. SPM behavior is therefore dependent on time and temperature. Thus SPM grains can, in principle, be detected us- ing variable temperature and/or variable time constant methods.

One technique for identifying SPM grains involves cooling the specimen to a low temperature, typically that of liquid nitro- gen (77K) or liquid helium (4K), and imparting an SIRM by

strongly magnetizing it with a high-magnetic field. The moment of the specimen can be monitored as it warms up. SPM grains at room temperature may be SD at colder temperatures, thus acquir- ing a cold IRM. This IRM will be lost as the grain passes through its "blocking temperature" and becomes SPM again. Comparison of the cold and room temperature measurements of IRM, then, will show significant SIRM loss on warm-up for specimens with appreciable SPM content. One should be aware, however, that larger grains sometimes are observed to undergo an abrupt loss of remanence on warming through what is termed the "Verwey transition", a temperature at which a magnetocrystalline anisotropy constant changes sign [e.g. Stacey and Banerjee, 1974], resulting in a loss of magnetic memory for grains whose remanence is blocked by magnetocrystalline anisotropy. The two effects can be distinguished however as the former is gradual and the latter is steplike.

A second method of SPM detection makes use of the fact that

SPM behavior is timescale-dependent. Grains that are nearly of SD size may have a useful remanence "half life" if measured over a short enough period of time. That is, larger SPM grains will in fact behave like SD grains. By using a variable frequency sus- ceptometer, we can change the time interval over which the specimen is measured. The presence of SPM grains will result in a lower apparent susceptibility at higher frequencies, because larger SPM grains will respond as SD grains for the shorter measuring time of the high-frequency mode, but the same grains will act as SPM grains for the longer measuring time of the low- frequency mode.

In this study we use magnetic susceptibility (•) and labora- tory-imparted remanences, SIRM and ARM, to try to understand variations in the grain size of the magnetic carder. This is pos- sible because the mineralogy of the magnetic carrier is simple: it is overwhelmingly controlled by magnetite [Hartl et al., 1993]. In the following we see how the rock magnetics provide substan- tial clues for understanding paleoenvironmental changes across the Eocene/Oligocene boundary.

Chemical and Magnetic Changes Across the Eocene/Oligocene Boundary at Site 522

Here we examine the portion of the Site 522 record spanning the Eocene/Oligocene boundary using inclination, SIRM, Z, and ARM (Fig. 2). The inclination data defines a magnetostratigra- phy correlated to magnetochrons C13n and CI3r [see Tauxe et al., 1984]. This chron boundary is estimated to be about 35 Ma in recent time scales [e.g. Cande and Kent, 1992]. The Eo- cene/Oligocene boundary itself is defined in the boundary strato- type at Massignano, Italy at "a level coincident with the last oc- currence (LO) of Hantkenina spp. and 14% of the stratigraphic distance downsection within Chronozone C13r" [Premoli-Silva et al., 1988, p.21 ]. The highest occurrence of Hantkenina spp. in core 522 is located at 137.2 m below sea floor (mbsf) [Poore, 1984] and is marked in Figure 2. Unfortunately, because the lowermost portion of C13r is missing in core 522, it is necessary to correlate core 522 to offset core 522A in order to estimate the

total core length of C13r so that a distance of 14% down the chron can be approximated. There are also several core breaks of unknown severity within C13r in core 522, adding to the uncer- tainty of this estimate. Doing so yields an estimated section length of 15 m for C13r and places the Eocene/Oligocene bound-

HARTL ET AL.' PALEOPRODUCTIVITY AND ROCK MAGNETICS 315

L. O. Hantkenina slain.

E 2.0 i

o 1.0

n, 0.0

Oligocene Eocene

I I I I

130 132 134 136 138 140 142 144 146

--- 2.5

E 1.5 •' 1.0 o

,,., (3.5

0.0 130 132 134 136 138 140 142 144 146

E 2.0 i

o 1.0

"" 0.5

n- 0.0 130 132 134 136 138 140 142 144 146

c13N c13R

80

40

0

-40

-80

130 132 134 136 1,3a 140 142 144 146

meters below seafloor

Figure 2, Magnetic inclination and three •sic rock magnetic parameters, saturation isothermal remanem magnetization (SIRM), magnetic susceptibility (Z) and anhysteretic reinanent magnetization (ARM), plotted against depth for tM Site 522 sediments. The Eocene/Oligocene boundary, as defined by the last occurrence (LO) of Hantkenina spp., is marked at the top of the Figure at 137 meters below seafloor.

ary at 136.2 mbsf. Because of the multiple uncertainties of this latter method, we choose the LO of Hantkenina spp. as our best estimate of the Eocene/Oligocene boundaEq at Site 522.

A number of rock magnetic changes occur near the Eo- cene/Oligocene boundary in core 522 which ailow us to divide the core into three zones; an Oligocene zone -above 137 mbsf, an Eocene zone below 139.2 mbsf, and a transitional zone (of very latest Eocene age) characterized by intermediate rock magnetic values, spanning from 139.2 to 137 mbsf. SIRM, ARM and Z are all greater for Eocene zone specimens (Figure 2), controlled in large part by the change in concentration. However, the ratios of these parameters to each other also change systematicaily. Plot- ting SIRM versus Z (Figure 3a)results in two quasi-linear tracks represented by Eocene zone samples and Oligocene specimens, with a lower ratio of SIRM to Z in Eocene sediments. Along

each track, changes are controlled predominantly by changes in concentration. The shift between tracks, however, suggests changes in magnetic grain size.

A lower SIRM-to-z ratio will result if the proportion of larger (> 10 gm) multidomain (MD) magnetite grains increases [e.g., Thompson and OldfieM, 1986] (Figure 1), because the presence of domain wails in the larger grains results in a greater increase in Z than SIRM, although this effect is small (see Figure 1). Alter- natively, a greater superparamagnetic (SPM) component, that is, an increase in the proportion of magnetite particles with grain size smaller than the SPM/SD threshold, can also result in lower

SIRM to g [Thompson and Oldfield, 1986] (Figure 1), because the SPM component strongly boosts Z while contributing nothing to remanene•. Another possibility is that very large changes in the paramagnetic and/or diamagnetic contribution due to a sig-

316 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

25

20-

15-

10-

5-

0 0

ß = Oligocene "= Eocene

I I I

10 15 20

x (10-Sm3/kg)

25

•-- 20

•<15 I

:•1o

o o

ß = Oligocene z• = Eocene

I- I I

10 15 20

x (10-Sm3/kg)

25

•-- 20

•<15 I

:• lO

0 I

ß = Ol[gocene • : Eocene

I I I I

10 1.5 20 2.5

SIRM (10-4Am2/kg)

0.6

0.5

0.4

0.3

0.2

0.1

0.0

SD

PSD a = Eocene

- [] = Oligocene

--

--

SP MD&SP

I I I I I

1 .o 2.0 3.0 4.0

Brc/Bc 5.0

Figure 3. (a) Saturation isothermal remanent magnetization (SIRM) versus magnetic susceptibility (Z). The Eocene group plots along a line of lower SIRM/z compared to Oligocene group. (b) Anhysteretic remanent magnetization (ARM) versus magnetic susceptibility (•). Eocene group is not nearly so distinct as in Figure 3a. (c) Anhysteretic remanent magnetization (ARM) versus saturation isothermal remanent magnetization (SIRM). No distinction between Eocene and Oligocene. (d) Hysteresis parameters for Site 522 specimens fall into two distinct groups, Eocene and Oligocene. Eocene specimens may contain a higher fraction of larger, multi-domain (MD) grain sizes, or, alternatively, a larger fraction of very fine, superparamagnetic (SPM) grains.

nificant change in clay mineralogy, carbonate content and/or sil- ica content could also produce this change. The difference in rock magnetics between Eocene and Oligocene sediments is also evident in plots of ARM versus • (Figure 3b), although much less distinctly. A plot of ARM versus SIRM (Figure 3c) does a very poor job in discriminating between Eocene and Oligocene specimens.

Figure 3d is a "Day" plot [Day et aL, 1977] of the ratios satu- ration magnetization (M.•) to saturation remanence magnetization

(Msr = SIRM) versus coercivity of remanence (Bcr) to coercivity (Bc). All specimens plot within the fine grain size end of the pseudo-single domain (PSD) field, but the Eocene sediments display lower Ms,/M, and higher Bc,?Bc ratios than their Oligocene counterparts. Like SIRM versus •, the hysteresis parameters in- dicate either a greater MD or SPM component for the Eocene sedimentsß

In addition to the rock magnetic measurements, we also de- termined carbonate content on 300 individual specimens from the

HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS 317

94-

92-

90- 88 86-

84-

82 • 150

17-

<15

o14

• 12

•11-

lO

Oligocene I Eocene

1,32 1,34 1,36 1,38 140 142

I i

144 146

I I I I I I I I I

30 132 134 136 138 i 40 142 144 1 46

24

< 20 o

'-•16

m 12 <

30 132 134 136 138 140 142 144 146

meters below seafloor

Figure 4. ARM/Z, SIRM/z, and percent CaCO3. Note the parallel increase in percent CaCO3 and SIRM/x. As in Figure 2, the Eocene/Oligocene boundary, as defined by the last occurrence of Hantkenina spp., is marked at 137 mbsf.

Eocene, Oligocene and transition zones and again found signifi- cant differences. Eocene sediments average 86.5% CaCO3, Oli- gocene sediments average 89.3% CaCO3, and the 137-139.2 mbsf transitional zone links these two linearly (Figure 4). The com- plimentary change in the noncarbonate fraction (NC) from Eo- cene values of 13.7% to Oligocene values of 10.5% represents a 23% decrease. In spectral analysis of magnetic susceptibility ver- sus depth across the Eocene/Oligocene boundary we found an in- crease in the wavelength of the dominant spectral component from -70 cm in the uppermost Eocene to -85 cm in the lower- most Oligocene (Figure 5). If this waveform represents the same temporal periodicity on both sides of the boundary, it would mean an increase in the sediment accumulation rate (SAR) of ap- proximately 21%. Thus a simple dilution of the NC component by an increase in CaCO3 accumulation rate appears to be ap- proximated. However, in order to match the nearest known or- bital period, that of the Earth's eccentricity (100 kyr), the SARs would need to be -0.7 and ~0.85 cm kyr 'l respectively, as op- posed to the approximately 1 cm kyr 'l reported by Hsa et al. [1984] and Mead et al. [1986]. Other methods used for SAR measurement, such as biostratigraphy and magnetostratigraphy, are hampered by core breaks just outside of this region and therefore do not have the resolution or accuracy to be helpful in further resolving this problem.

In the carbonate record, we note that the percent CaCO3 rise from Eocene to Oligocene values depicted in Figure 4 is initiated

0.5

0.4

o.5

0.2

0.1

0.0

0.00

85 cm (01igocene) I

_

0 cm (Eocene)

,'x/ ' I •

•. ' \\

0.02 0.04 0.06

Frequency, cm-1

Figure 5. Spectral analysis of the magnetic susceptibility changes with depth in the uppermost Eocene and the lowermost Oligocene of Site 522. This shift of the major peak from 70-cm to 85-cm wavelength may indicate an increase in sediment accumulation rate across the Eocene/Oligocene boundary at this site.

318 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

by a conspicuous drop to some 83% CaCO3 at 139.2 mbsf, 2 m below (~ 200 kyr before) the Eocene/Oligocene boundary as de- termined by the LO of Hantkenina spp. [Poore, 1984]. Also plotted in Figure 4 are the ratios SIRM/x and ARM/x against a common depth scale. Note the concurrent shifts to higher values for SIRM/x, ARM/x and percent of CaCO3 over the transitional interval from 139.2 mbsf to 136 mbsf. We will return to these

observations and compare them to stable isotope records in the "discussion and conclusions" section.

Combining rock magnetics with carbonate data, we find it in- structive to consider how the ratios SIRM/NC and X/NC vary across the Eocene/Oligocene boundary. By normalizing both to their mean values (Figure 6) it is clear that the average value of SIRM/NC does not change but that z/NC is distinctly higher in the Eocene sediments. The unchanging nature of SIRM/NC sug- gests that the noncarbonate portion contains a fairly constant fraction of remanence carrier over time. This would argue against large changes in the MD fraction of the magnetic phase as such changes would be expected to effect both X/NC and SIRM/NC (see Figure 1).

To check if the change in X/NC is due to a change in the par- amagnetic fraction, we determined the paramagnetic slope from hysteresis loop "tails" for each specimen, normalized by SIRM, and found no correlation. In addition, Fourier transform infrared

(FTIR) and X ray diffraction analyses of the noncarbonate por- tion indicate no detectable change in clay mineralogy across the

boundary. We also calculate that the very slight increase in the Oligocene sediments' diamagnetism due to higher CaCO3 is not nearly sufficient to account for observed change in X/NC. Thus diamagnetic and paramagnetic changes can be ruled out as the source of the change in X/NC.

As there are no apparent changes in remanences, diamag- netism or paramagnetism sufficient to explain the changes in X, we now examine the possibility of changes in superparamagnetic behavior by measuring susceptibility over a range of frequencies. The susceptibility of our small specimens (1 cm 3) was too weak to measure with a standard Battington variable frequency suscep- tometer, but not too weak for the Lakeshores susceptometer be- longing to the Institute for Rock Magnetism at the University of Minnesota. We measured susceptibility across a frequency range spanning from 10 Hz to 10 kHz (see Figure 7). All specimens exhibited decreased susceptibility at higher frequencies, indicat- ing the presence of superparamagnetic grains, but we could find no discernible difference between the Eocene and the Oligocene groups.

As a further test, we imparted an IRM at liquid nitrogen tem- perature (77K) and allowed 130 selected Eocene and Oligocene specimens to warm up in the magnetometer, measuring the rema- nence and temperature at 3-s intervals. The resulting curves of magnetization versus time (Figure 8; 20 examples shown) show a small but distinct difference between the Eocene and Oligocene groups; the Eocene group loses more of its magnetic moment on

1.4 Oligocene Eocene

1.3

1.2

1.1

Oz 1.0 0.9

0.8

0.7

0.6 • 132 134 136 138 140 142

1.4

1.3

1.2

•1.1

1.0

0.9

0.8

0.6 132 134

I i i i

136 138 140 142

meters below seafloor

Figure 6. Ratios of SIRM/noncarbonate (SIRM/NC) and X/noncarbonate (x/NC), normalized to their mean values (dashed line at y = 1.0), versus depth. Note that SIRM/NC exhibits no consistent trend, whereas x/NC does. These results support a greater SPM contribution from Eocene sediments. The Eocene/Oligocene boundary, as defined by the last occurrence of Hantkenina spp., is marked at 137 mbsf.

Frequency Dependence of Susceptibility

•.3

0.9

0.8

0.7

1 0 1 O0 1000 1 04

Frequency, Hz

Figure 7. Variable frequency data from representative Oligocene (open symbols) and Eocene (filled symbols) specimens. Although them is a substantial frequency dependence in both groups, suggesting SPM behavior, them is no detectable difference between the two groups.

319

1.00

095

IRM imparted @ -196øC

Sample warmup in magnetometer

0.90

._N 0.85

E o

z

O.8O

0.75 u U Oligocene

•0 0 0 •

0.70 Eocene

0 50 100 150 200 250 300 Time (seconds)

Figure 8. Curves of IRM intensity, normalized to initial values, versus time and temperature (temperatures are indicated above x axis). Specimens were subjected to a field of 0.6 T while bathed in liquid nitrogen (temperature of- 196øC), then transferred to a cryogenic magnetometer and measured every 3 s while warming for 300 s. The Oligocene group (dashed lines) retains an average of 73.5% of cold IRM, whereas the Eocene group (solid lines) average 72%. The greater loss of moment for the Eocene group is consistent with a greater proportion of SPM grains. Both groups experienced further loss of IRM when brought fully to room temperature.

320 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

warm-up than the Oligocene group. Note that the curves show no evidence of a Verwey transition which would be expected for a population containing a significant proportion of multidomain magnetite grains [Stacey and Banerjee, 1974]. The loss of rema- nence on warm-up is likely due to small magnetite grains that are stable remanence-carrying SD grains at 77K, but which are transformed to thermally unstable, nonremanence-carrying SPM grains as the temperature rises above each grain's blocking tem- perature.

Figure 9 shows the percent of IRM loss upon warm-up against IRM/z. The correlation between the two variables is striking, and further supports the hypothesis that a greater superparamag- netic component in the Eocene sediments is the cause of the SIRM/Z shift. Also apparent is a small subgroup of Eocene specimens that do not fit the trend, indicating a greater complex- ity to the story than our simple hypothesis offers.

As a final check of the rock magnetics, we generated and re- corded hysteresis loops at 20 K for selected specimens and com- pared these loops to those measured at room temperature. The presence of superparamagnetic grains will cause a "fattening" of hysteresis loops with decreasing temperature as both remanence, Msr (= SIRM) and coercivity, Bc, increase when superparamag- netic grains become stable single domain grains at low tempera- ture. Figure 10 shows this effect for one representative Eocene specimen, for which the dashed line is the trace of the hysteresis loop at 298K and the solid line is the "fattened" loop at 20K. For each specimen we measured Bc (x axis crossings) and Msr (y axis crossings) at both temperatures and determined the change in

these parameters with temperature. Figure 11 is a plot depicting these changes, with coercivity changes on the x axis and satura- tion remanence changes on the y axis (both expressed as a ratio of the 20-K value over the 298-K value). Although there is overlap of the Eocene and Oligocene groups, it is also clear that taken as a whole, the Eocene specimens undergo greater change with temperature, again implying a greater proportion of super- paramagnetic grains.

In summary of our rock magnetic measurements, we submit that although the variable frequency test for superparamagnetism yielded no difference between the two groups, the IRM warm-up curves and the hysteresis results offer significant evidence of a change in the proportion of the superparamagnetic content of the specimens across the Eocene/Oligocene boundary. We note that Maher [1988] obtained similar negative results using dual- frequency susceptibility, but positive results using low tempera- ture methods for detection of superparamagnetic magnetite. The alternative hypothesis of a change in grain size of much larger MD magnetite grains is problematic in a number of ways: (1) it is not supported by the lack of observable Verwey transition in the IRM warm-up curves of Figure 8; (2) the "Day" plot (Figure 3c) indicates an average small PSD-size range much closer to SD size than the MD size; (3) the variable frequency susceptometer results indicate a population which contains a substantial super- paramagnetic portion, which, assuming a lognormal distribution, argues for an overall smaller magnetic carrier size; (4) MD grains do not explain the observed trends in the IRM warm-up results or hysteresis loop changes with temperature; and (5) a change in

1.60E-04 Site 522 IRM Warmup Loss vs. Irm/x

1.50E-04

1.40E-04

:• 1.30E-04

1.20E-04

1.10E-04

FI

12 El Oligocene Eocene

1.00E-04 I I ,• ! • • • I 31 32 33 34 35 36 37 38 39 40

% IRM Loss on Warmup (77k => 298 K)

Figure. 9. Percent loss of IRM after each was subjected to a 0.6 T field at 77K, measured for magnetization, then allowed to warm to room temperature (298K) in a field-free environment and subsequently remeasured.

HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS 321

Eocene sample 56; 20 and 298 K

o.o-

-0.,5 -

1.0- I i

-400 -200 0 200 400

Field, mT

Figure 10. Hysteresis loops for a representative Eocene specimen measured at two different temperatures, 298K (dashed line) and 20K (solid line). The y axis crossings mark Msr (normalized by Mr) (M), and the x axis crossings define Bc.

MD grainsize is not consistent with the observed change in z/NC and lack of change in SIRM/NC.

Finally we note that ARM versus Z, which has been advocated as a rapid method for distinguishing changes in magnetic grain size [Banerjee et al., 1981; King et al., 1982], yields a rather in- conclusive plot (Figure 3b) with only a very subtle trend of gen- erally lower ARM/• for the Eocene. If we are correct in inter- preting a greater superparamagnetic fraction for the Eocene, the lower ARM/• ratio for these sediments (Figure 4) is opposite to the sense normally expected for a fining of magnetic grain size. We believe this demonstrates that for grain size populations straddling the SD/SP size transition, ARM/• will be a very unre- liable indicator of magnetic grain size changes (about which King et al. [1982] caution the reader). The greater scatter of the ARM/• plot when compared to SIRM/z (Figures 3a and 3b) may be explained by noting that the ratio ARM/• changes rapidly and nonuniquely with changes in magnetic grain size if a significant proportion of the grains are near the SD/SPM transition, whereas SIRM to • is less sensitive for such a population, because SD magnetite grains contribute considerably less (proportionately) to SIRM than to ARM.

Discussion and Conclusion

The increase in SIRM/Z across the Eocene/Oligocene bound- ary at Site 522 is most likely attributable to a decrease in the pro-

1.6

E1.4 o

1.2

1.1

r-i El

A []

r'lA

"=Eocene

D=Oligocene

I- ! I I I, I , I I I I

1.80 1.90 2.00 2.10 2.20

Bc20//Bc2•js Figure 11. As a measure of the change of each hysteresis loop with temperature (see Figure 9), we plot the change in magnetization with temperature as the ratio of Mr at 20K to Mr at 298K on the y axis (both normalized by Mr), and the change in coercivity as the ratio of Bc at 20K to Bc at 298K on the x axis.

322 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

portion of the finest grain size (superparamagnetic fraction) of magnetite in the early Oligocene. The fact that $IRM/NC does not change implies that only the superparamagnetic portion has been reduced in the Oligocene, that is, magnetite of average di- ameter less than 0.025 gm. The smaller proportion of this grain size is likely a result of incipient dissolution, whereby only the smallest grains of magnetite are destroyed.

We now compare our rock magnetics to oxygen and carbon isotopes from one planktonic and two benthic foraminifera spe- cies from Site 522 using data of J. Zachos, T. Quinn and K. Lohman (unpublished data, 1994) and Oberhiinsli et al. [ 1984] in Figure 12. Note the parallel trend in •5•3C to SIRM/g (and per- cent CaCO3, see Figure 4), particularly over the transitional in- terval from 139.2 to 136 mbsf. Like percent CaCO3, õ13C gradually increases from low values at core depths below 139 mbsf to much higher values at 133 mbsf. Furthermore, the •5•3C record also displays an excursion to lowest values at 139.2, ex- actly where we found an excursion in the CaCO3 record and also where these parameters begin shifting to higher values. These re- cords suggest a common oceanographic control. An increase in the ratio of planktonic to benthic foraminiferal tests and a de- crease in shell fragmentation have also been observed at this point in the record, indicative of a lowering of the lysocline [Boersma, 1986]. Thus, while the chemical stability of magnetite is slightly decreased, the stability of CaCO3 is simultaneously en- hanced. An increase in productivity could explain these changes; the increase in carbonate sedimentation due to higher productiv- ity in the overlying surface waters will tend to depress the lyso- cline, at least locally, and cause increased carbonate accumula- tion. At the same time, the accompanying increased organic car- bon rain will promote iron and sulfate reduction resulting in in- cipient dissolution of magnetite [Tarduno, 1994]. This "reduction diagenesis" of magnetite has been documented and investigated in several marine sediment cores [Karlin, 1990; Karlin and Levi, 1983, 1985; Kobayashi and Nomura, 1972; Canfield and Berner, 1987; Charmell and Hawthorne, 1990].

Noted previously, a straightforward interpretation of the spec- tra of •; with depth across the Eocene/Oligocene boundary indi- cates an increase in carbonate accumulation of approximately 20%. The proportional decrease in the NC component is also consistent with this interpretation. The lack of discernible change in the NC mineralogy (and the magnetic remanence car- tier contained within it) indicates that the NC component was de- posited relatively continuously throughout this time and was pro- gressively diluted by a rise in carbonate accumulation resulting from increased productivity and preservation in the earliest Oli- gocene. Organic carbon content of Site 522 sediments [Hsii et al., 1984] is very low, 0.1% or less, typical of marine sediments of SAR less than 10 cm kyr '• [Broecker and Peng, 1982] in which labile organic carbon is quickly taken up in various reac- tions, thereby destroying direct organic carbon evidence of pro- ductivity.

Significant shifts in •j13C values such as we observe in the Site 522 record may reflect carbon reservoir shifts such as the Mio- cene global "Monterey excursion" of Vincent et al., [1980], or basin-to-basin carbon transfers or marine-to-terrestrial isotopic fractionation of the late Pleistocene [Shackleton et al., 1983; Shackleton, 1977]. In addition to the Eocene/Oligocene •3C shift at Site 522, shifts to heavier carbon isotopes have been found at Atlantic DSDP Sites 292, 366, 529, 516 [Grazzini and

Oberhansli, 1986], Site 563 [Miller et al., 1985], and a number of Indian and Southern Ocean sites including Ocean Drilling Program (ODP) Site 748B [Zachos et al., 1992], Site 689 [Kennett and Stott, 1990], and Sites 744B and 749B [Mackensen and Ehrrnann, 1992]. Thus, it appears to be a global shift and not one due to a basin-to-basin type transfer. Most of these rec- ords indicate a transient õ•3C change, with a much more long lived õ•80 change. The Eocene/Oligocene õ•3C change in water chemistry, if correlated with Pleistocene and present day relation- ships between nutrient concentrations and õ•3C [Broecker and Peng, 1982], would argue for a transient influx of nutrient- depleted water both shallow and deep. More plausible perhaps is the above mentioned marine-to-terrestrial reservoir type mecha- nism. Whatever the root cause of the õ•3C signal is, it is indica- tive of a change in water mass chemistries over much of the globe, a change also expressed in carbonate sediment accumula- tion, lysocline depth and rock magnetics at Site 522.

The rapid steplike shift in õ•80 to higher values at 134 mbsf, one of the largest õ•80 shifts for the entire Tertiary, has been in- terpreted as evidence of an abrupt cooling of the global deep ocean [Shackleton and Kennett, 1975; Kennett and Shackleton, 1976; Kennett and Barker, 1990] and the first major Antarctic ice buildup [Matthews and Poore, 1980] and most probably marks the initiation of intermittent continental glaciation of the Antarc- tic continent [Miller et al., 1991a]. This step in õ•80 occurs some 5 m higher (about 500 kyr later) than the beginning of the above noted changes at 139 mbsf, although it is foreshadowed by a more gradual shift in one õ•80 record, that of Cibicidoides, starting at 139 mbsf (Figure 11). Thus, the global õ•80 step at 134 mbsf seems to be firmly linked to the low CaCO3 and õ•3C event at 139.2 mbsf, which marked the beginning of major changes in CaCO3, •3C and magnetite dissolution at Site 522.

There are few other Eocene/Oligocene boundary records at present that have been investigated at the resolution necessary to find and corroborate the short-duration event that we observe in

the Site 522 record at 139.2 mbsf. There are certainly no concen- trations of microtecktites, iridium, or other traces of a bollide im-

pact or any other catastrophic event associated with the Eo- cene/Oligocene boundary (the Noah American microtecktite strewn field once thought to be associated with it [Ganapathy, 1982] has been shown to be some 1.5 myr older [Keller et al., 1987; Miller et al., 1991b]). Moreover, interpreting global events from a single record is risky, to say the least. However, the short-duration excursion to low CaCO3 and õ•3C values at 139.2 mbsf, representing a period of perhaps only a few hundred years, is directly linked to the globally observed õ•80 increase some 500 kyr later. We find it quite plausible that the event at 139.2 mbsf records some unusual occurrence in which the global ecology was in some way stressed, or at least altered, afterwhich it rebounded in a new fashion, moving to a colder equilibrium in the early Oligocene. This "rebound" appears to be linked to a rise in biologic activity at Site 522.

The sensitivity of rock magnetic measurements to environ- mental change is remarkable in the sediments of Site 522. Moreover, if we are correct in our conclusion that the dissolution

of very fine-grained superparamagnetic magnetite is due to in- creased organic carbon influx from higher Oligocene productiv- ity, then it follows that rock magnetics can be a useful tool in in- vestigating a process which often leaves little other trace in the record.

HARTL ET AL.' PALEOPRODUCTIVITY AND ROCK MAGNETICS 323

3.0

2.5

1.$

1.0

Oligocene Eocene

_ x= Catalas•ctroa: ctiss.

ß

i I I I I I I I I

130 132 134 136 138 140 142 144 146

2.5

2.0

1.5

1.0

0.5

0.0 i I I i i i i i i

130 132 134 136 1.38 140 142 144 146

16

E 15

o 14

n'12 --

11

130 132 134 136 138 140 142 144 meters below seafloor

146

Figure 12. Site 522 benthic foraminifer Cibicidoides and Gyroidina isotope data of J. Zachos, T. Quinn, and K. Lohman, (unpublished data, 1994) and planktonic foraminifera Catapsydrax dissimulis of Oberhansli et al., [1984]. The low "excursion" value of õ•3C at 139.2 mbsf is matched by a similar low in the percent CaCO3 record of Figure 4. The rising trend of õ•3C from 139 to 132 mbsf is also matched in the carbonate record as well as in the SIRM/x of Figure 4. This trend can also be seen (but less obviously) in the õlSO record (especially Cibicidoides). The large steplike õ•SO increase at 134 mbsf is a globally observed early Oligocene event thought to record a major Tertiary threshold in climate degradation (see text for discussion). The Eocene/Oligocene boundary, as defined by the last occurrence of Hantkenina spp., is marked at 137 mbsf.

324 HARTL ET AL.: PALEOPRODUCTIVITY AND ROCK MAGNETICS

Acknowledgments. The authors would like to thank Steve DiDonna at Scripps and Paula Weiss at the ODP East Coast Repository for technical assistance, as well as Jim Marvin, Chris Hunt and Bruce Moskowitz and the Institute for Rock Magnetism for the use of the IRM facilities, as well as for thoughtful and excellent advice. We also thank Jim Zachos, Ten'y Quinn and K.C. Lohman for access to their excellent isotope data, and John King, Dennis Kent and Ken Miller for helpful comments. This work was supported by NSF grants OCE 89-17252 and OCE92-17966. Funds for paleomagnetic equipment at Scripps were provided by the W.M. Keck Foundation. The IRM is funded by grants from the W. M. Keck Foundation, the National Science Foundation and the University of Minnesota. L. Tauxe gratefully acknowledges partial support from the University of Utrecht.

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(Received November 10, 1993; November 29, 1994; accepted November 29, 1994)